If you select the %EZ_Subclass event for a control in the designer (or use EZ_Subclass command in hand written code) you can subclass a control and it will now get the %EZ_Subclass event.
There are a number of support commands which make working with %EZ_Subclass event easier such as:
EZ_SubClassDefault
EZ_GetSubClass
EZ_SetSubClassRVal
EZGUI actually subclasses all new controls (to provide some features not normally available), but each control has a subclass mode it is in and this determine what events get through.
EZGUI has multiple subclass modes internally, including the Visual Designer mode ( enables drag and drop and resizing of controls with the mouse).