The normative source for all terminology used across the Simulation Fidelity Rating framework.
This page contains the authoritative definition for each term used in classification, evaluation, and determination. When a term defined here appears in any other framework document, it carries the meaning established here. No other definition of these terms is normatively binding within the SFR framework.
These definitions are normative. They apply to all documents within the SFR framework, including normative framework documents, informative reference documents, commentary documents, and historical reference documents. Where a term defined here is used in any framework document, it carries the meaning established on this page.
These definitions do not govern usage outside the SFR framework. They do not supersede definitions established by other standards bodies for their own instruments. They are specific to the SFR classification and evaluation context.
If a term appears in a classification determination, it is governed by this page.
A simulation system is a structured technical environment that attempts to replicate selected physical conditions of a real-world vehicle dynamics scenario for the purpose of training, evaluation, or research. Within this framework, simulation validity is determined by the accuracy of physics representation, the correctness of sensory delivery, and the structural relationship between vehicle physics state and participant experience. A simulation system is not defined by its visual quality, enclosure design, number of motion actuators, or subjective immersiveness.
The loop is the closed, continuously updating cause-and-effect chain between the vehicle physics model, sensory signal delivery, participant perception, control input, and resulting vehicle state change. The loop is complete only when each element of the chain is causatively connected to the preceding element in the correct sequence and timing relationship. The loop is broken whenever any element is decoupled, approximated, replaced by a non-causative substitute, or delivered outside a valid timing relationship.
An in-the-loop simulation system is one in which the vehicle physics model directly governs the motion state experienced by the participant, with independent degrees of freedom resolved at the vehicle's center of mass and synchronized across all sensory systems. A system qualifies as in-the-loop only when it satisfies all six requirements defined in the In-the-Loop Standard and meets all three fundamental criteria defined in What Is The Loop? The designation is structural. It is not granted by appearance, motion quantity, or self-description.
A surface-level simulation system is one that produces motion or sensory output intended to approximate the appearance or feel of a real vehicle response, but in which the motion is applied after the physics event rather than derived directly from it, is not resolved at the vehicle's center of mass, or is delivered through mechanically coupled axes that cannot produce independent degree-of-freedom responses. Surface-level systems produce motion but do not satisfy the structural requirements for in-the-loop status. The presence of motion does not constitute in-the-loop classification.
An out-of-the-loop simulation system is one in which the participant's experience does not include any mechanically delivered motion derived from the vehicle physics state. The participant may receive visual or audio output from the simulation but is not positioned within a closed sensory feedback chain. Out-of-the-loop systems include static simulators, screen-based environments, and any configuration in which no physics-derived motion stimulus reaches the participant. Out-of-the-loop is a structurally distinct classification from surface-level; the two categories must not be combined in determinations.
Causative accuracy is the requirement that each sensory cue delivered to the participant must originate directly from the live physics event it represents. A cue that is scripted, triggered by visual information, derived from pre-recorded motion data, approximated as a post-physics effect, or layered as a decorative motion profile does not meet the causative accuracy requirement. Causative accuracy is Criterion A of the three fundamental criteria required for in-the-loop status. It is supported by Requirement 1 (Physics-Driven Motion) and Requirement 2 (Center-of-Mass Reference) of the In-the-Loop Standard.
Temporal coherence is the requirement that all sensory channels — vestibular, visual, haptic, audio, and control feedback — arrive at the participant in the correct sequential relationship and within the neurological detection threshold applicable to each channel. Violation occurs when any channel arrives out of sequence relative to another, or when any channel is delayed beyond its neurological detection threshold. Temporal coherence is violated regardless of the accuracy of individual channels when the inter-channel timing relationship is incorrect. Temporal coherence is Criterion B of the three fundamental criteria required for in-the-loop status. It is supported by Requirement 3 (Independent DOF) and Requirement 4 (Real-Time System Coherence) of the In-the-Loop Standard.
Human response relevance is the requirement that the sensory cue delivered to the participant must be of sufficient fidelity and signal strength to alter natural control behavior. If the participant's control corrections are governed primarily by visual input rather than physical sensation, the human response relevance criterion is not met. Signal strength is not optional. Human response relevance is Criterion C of the three fundamental criteria required for in-the-loop status. It is supported by Requirement 5 (Vestibular Relevance) and Requirement 6 (Training Validity) of the In-the-Loop Standard.
Independent degrees of freedom are rotational and translational motion axes that each operate through a mechanically and computationally separate control pathway, such that any single axis can be actuated independently without altering the instantaneous state of any other axis. Axes that are mechanically coupled or software-blended — such that actuation of one necessarily affects the output of another — are not independent. Independent degrees of freedom are a structural requirement for in-the-loop status and a necessary condition for temporal coherence, because axis coupling corrupts the independent timing of compound motion events.
The center of mass, as used in this framework, refers to the physical point within a vehicle's mass distribution about which all rigid-body rotational motion is correctly represented under the equations of rigid-body dynamics. For a simulation system to deliver causatively accurate motion, all rotational and translational motion must be resolved relative to this reference point. Simulation motion that rotates about any other geometric point will produce inertial cues that do not correspond to the vehicle's actual rigid-body behavior. Failure to resolve motion at the center of mass constitutes a violation of both causative accuracy and center-of-mass reference requirements.
The following six definitions apply to terminology used in the Human Outcomes Layer of the SFR framework. These definitions govern the meaning of each term when it appears in any Human Outcomes Layer document. They do not modify or supersede the ten physics and classification definitions above; they extend the framework's canonical terminology into the domain of neurological processing and training outcomes.
Sensory coherence is the condition in which vestibular, proprioceptive, and visual inputs delivered by a simulation environment are physically consistent with one another and with the motion state being simulated. Sensory coherence exists when no channel delivers information that contradicts another channel. It is a necessary condition for In-the-Loop classification and a prerequisite for the neurological processing conditions that support training transfer. Sensory coherence is distinct from sensory accuracy: a system may deliver mutually consistent inputs that are nonetheless inaccurate representations of the intended physics state. Coherence requires internal consistency across channels; accuracy requires correspondence to real-world physics. In-the-Loop classification requires both.
Training transfer is the degree to which motor patterns, neurological adaptations, and behavioral responses acquired during simulation are expressed in the real-world environment the simulation is intended to represent. High training transfer means the nervous system treats simulation-acquired adaptations as applicable in the target environment. Low training transfer means the adaptations formed in simulation are context-specific to the simulator and do not generalize. Training transfer is not the same as simulator performance. A participant may perform well in a simulator while acquiring adaptations that do not transfer, and may perform poorly in simulation while acquiring adaptations that transfer fully. Simulator performance and training transfer are independent variables.
Neurological loading is the total processing demand placed on the nervous system during a simulation session, comprising sensory integration cost, motor coordination demand, executive cognitive load, and conflict-resolution overhead. Neurological loading is session-specific and cumulative within a session. It is not inherently harmful — appropriate loading is the mechanism of productive training. Neurological loading is distinct from Compensatory Demand: loading is the total demand on the nervous system from all sources during a session; Compensatory Demand is the subset of that loading attributable specifically to resolving sensory conflict arising from physically incoherent simulation inputs. An In-the-Loop system produces loading without generating Compensatory Demand. A Surface-Level system may generate both.
Compensatory demand is the component of neurological loading attributable to resolving sensory conflict arising from physically incoherent simulation inputs. When the vestibular, proprioceptive, and visual channels deliver contradictory information, the nervous system allocates processing resources to identifying and resolving the conflict. This resolution constitutes compensatory demand. Compensatory demand consumes neurological reserve without producing task-relevant motor or cognitive adaptation. It is the mechanism by which Surface-Level simulation may produce neurological loading without producing training transfer. Compensatory demand is not the same as task difficulty or cognitive challenge: a demanding In-the-Loop session may produce high neurological loading with low or zero compensatory demand, while a low-demand Surface-Level session may generate non-trivial compensatory demand. The two variables are independent.
Terminology note: The term "Compensation Demand" appears in supplementary reference documents predating this definition. The canonical term is "Compensatory Demand." Both terms refer to the same concept.
Reduced neurological reserve is a condition in which an individual's available neurological processing capacity is diminished relative to the baseline demands of a given simulation environment, due to neurological injury, progressive neurological disease, acute illness, aging-related vestibular or cognitive decline, fatigue, or cumulative exposure to sensory conflict. Individuals with reduced neurological reserve may be at elevated risk in Surface-Level simulation environments because the compensatory demand generated by sensory conflict may constitute a larger proportion of available reserve than it would in a neurologically typical participant. Reduced neurological reserve is not a clinical diagnosis within this framework. It is a framework descriptor for a condition relevant to simulation-tier selection decisions in medical, rehabilitation, and high-risk training contexts. Clinical determination of reserve status remains the responsibility of qualified clinical decision-makers.
Vestibular priority is the principle that vestibular information is typically available before visual confirmation during motion events and therefore occupies a primary role in anticipatory control, sensory weighting, and reaction timing. The vestibular system detects the onset of a motion event through the otolith organs and semicircular canals before the visual system can confirm the event through scene-motion correlation. This temporal precedence means the nervous system's initial response to a motion event is based primarily on vestibular input. In simulation environments that do not deliver causatively accurate vestibular input, the vestibular priority mechanism receives incorrect or absent information at the moment of greatest influence on initial motor response. This has implications for reaction time preservation and for the validity of training that depends on accurate anticipatory control.
These sixteen definitions constitute the complete normative terminology base of the SFR framework at version 0.9 Draft. Definitions 1–10 cover the physics and classification layer. Definitions 11–16 cover the Human Outcomes Layer. No classification determination, evaluation result, or framework document may apply these terms with a meaning inconsistent with those defined here.
If an apparent conflict exists between a definition on this page and language in another framework document, this page takes precedence. Discrepancies should be reported through the public review process described in the Governance Framework.