As a statechart grows, it may easily become too big to still give a comprehensive overview of the whole model. Subdiagrams come as a solution. Basically, you can “fold away” a composite state into a subdiagram.
When the Extract Subdiagram refactoring is executed on a composite state, all containing regions are extracted into a separate diagram. The composite state no longer clutters the diagram with all its internal details, but instead appears almost like a normal state. The only difference is a small decorator icon in the lower-right corner of the state, indicating the existence of a subdiagram. When you hover over this decorator with the mouse cursor, you’ll see a small preview of the subdiagram’s content.
Extracting a subdiagram creates entry and exit points in the subdiagram as needed.
Subdiagram pop-up window
A click on the decorator opens the subdiagram in a separate editor tab. The breadcrumb at the top allows easy navigation throughout the hierachy levels.
Using the inlining subdiagram refactoring, you can turn a subdiagram back into the composite state.