The twostateVariable type has the following optional properties

• TransitionTime specifies the time when the current state was entered.
• EffectiveTransitionTime specifies the time when the current state or one of its sub states was entered. If, for example, a LevelAlarm is active and – while active – switches several times between High and HighHigh, then the TransitionTime stays at the point in time where the Alarm became active whereas the EffectiveTransitionTime changes with each shift of a sub state.
• EffectiveDisplayName from the StateVariableType is used if a state has sub states. It contains a human readable name for the current state after taking the state of any SubStateMachines in account. As an example, the EffectiveDisplayName of the EnabledState could contain “Active/HighHigh” to specify that the Condition is active and has exceeded the HighHigh limit.

The alarm model utilise TwoStateVariables in a number of places, and for all place where it make senses additional conformanceUnits should be defined indicate that the specific twoStatevariable (say Enable) expose the optional property All three may not be required in all cases (i.e. if the given state does not have SubStates defined ten it might not need effectiveTRansitionTime or EffectiveDisplayName.