How to create a UML composite state element using Sparx Systems' Enterprise Architect application:
- In a state machine diagram, create a new state element.
- Select the state element's context menu item Advanced / Composite Element.
The selected state element is converted into a composite state element (the image has a infinity symbol) with its own state machine diagram (check the Project Browser). Now you can draw a transition line to and from this composite state and include it in state transition tables.
Annoyingly, Enterprise Architect's on-line help describes a composite element but doesn't show to make one!