For example, it is possible to group events together to reflect sequential information. Also, one can imagine that certain layers should be grouped together to reflect the fact that they all perform a similar function, and should be treated as a group. Similarly, units within a layer can be divided into subgroups that might reflect different parameter settings, etc.
Substructure is defined by creating subgroups of objects.
Thus, everywhere the user has the opportunity to create an object of a given type, they also have the opportunity to create a subgroup of such objects. For more information, see section 8.2 Groups .