States can be decomposed into sub-states using Hierarchy. This is extremely useful for adding a further level of detail to the behavior.
At the same time hierarchy can be used to represent levels of interrupt and priority to transitions.
When hierarchies are used in order to the visualize interrupt levels, the respective super-states need not be named, unless a related timeout expression is used. Obviously when hierarchies are used for state decomposition purpose, it is essential that the respective super-states be named accordingly.