(1) The set of all modes specified for all components of a system instance form a set of concurrent mode state machines, whose composite state is called system operation mode (SOM). The set of possible SOMs is the cross product of the sets of modes for each component. That is, a SOM is a set of component modes, one mode for each component of the system. The initial SOM is the set of initial modes for each component.
(2) The set of possible SOMs can be reduced by taking into account mode combinations that are not reachable. For example, if a component instance with modes is itself not active under certain modes of one of its containing components, then these mode combinations can be excluded. Similarly, of a component inherits modes from a containing component, then only the mode combinations of the containing component need to be considered. Finally, reachability analysis of modes taking into account mode transition conditions may further reduce the set of SOMs to be considered.
(3) The discrete variable Mode denotes a SOM. That is, the variable Mode denotes a compound discrete state that is defined by the mode hybrid semantic diagrams for each component instance in the system. Note that the value of Mode will in general change at various instants of time during system operation, although not in a continuous time-varying way.