A process is the work someone (or something) does to accomplish an objective. A process represents a sequence of steps in the form of a graph.
A SubProcess (Embedded, Event, or AdHoc) has parts that are modeled in a child-level process, a process with its own step flows and start and end states.
A CallActivity is a reusable activity. It's behavior is like a sub process except the sub process is not embedded in the parent, instead it's defined as it's own process and can be reused in other process models.
The called_element property can be a string representing the id of the called process or an expression that evaluates to the id.