In a hierarchical state chart, more than one state can be active at the same time. If the state chart is in a simple state that is contained in a composite state, then all the composite states that either directly or transitively contain the simple state are also active. Furthermore, since the state chart and some of the composite states in the hierarchy may be orthogonal (that is, contain multiple regions), the current active state is represented by a set of trees of states starting with the top-most states of the root regions down to the innermost active substate. These state trees are referred to as a state configuration. Except during transition execution, the following invariants always apply to state configurations:
•If a composite state is active and not orthogonal, at most one of its substates is active.
•If the composite state is active and orthogonal, all its regions are active, with at most one substate in each region.