To create subactivities from action blocks, right-click the action block and select Create Sub Activity Diagram. IBM® Rational® Rhapsody® creates a subchart containing the former contents of the action block.
Consider the action block shown in the following figure. When you create a subactivity for the action block, the parent state changes to that shown in the following figure.

The icon in the lower right corner indicates that the action block has a subchart.