The compound type of Activity is called a SubProcess. The graphical difference between a Task and a Sub-Process is that a Sub-Process has a "plus sign" placed in the lower center of the shape, indicating that it can be opened up for more detail.
A Sub-Process represents a compound Activity. In this sense, "compound" means that its work is broken down into a finer level of detail (i.e. another Process). Thus, a "hierarchical" Process model is possible with different degrees of detail at each level. There are two graphical representations of Sub-Processes: • Collapsed • Expanded