A compound transition represents a “semantically complete” path made up of one or more transitions, originating from a set of states (as opposed to pseudostate) and targeting a set of states. The transition execution semantics described below refer to compound transitions. In general, a compound transition is an acyclical unbroken chain of transitions joined via join, junction, choice, or fork pseudostates that define a path from a set of source states (possibly a singleton) to a set of destination states (possibly a singleton).
For self-transitions, the same state acts as both the source and the destination set. A simple transition connecting two states is therefore a special common case of a compound transition. The tail of a compound transition may have multiple transitions originating from a set of mutually orthogonal regions that are joined by a join point. The head of a compound transition may have multiple transitions originating from a fork pseudostate targeted to a set of mutually orthogonal regions.
In a compound transition, multiple outgoing transitions may emanate from a common junction point. In that case, only one of the outgoing transitions whose guard is true is taken. If multiple transitions have guards that are true, a transition from this set is chosen. Note that in this case, the guards are evaluated before the compound transition is taken.
In a compound transition where multiple outgoing transitions emanate from a common choice point, the outgoing transition whose guard is true at the time the choice point is reached, will be taken. If multiple transitions have guards that are true, one transition from this set is chosen. If no guards are true after the choice point has been reached, the model is ill formed.