A container holds the set of steps that comprise a subprocess.

A container can be minimized to display as an icon within the process, and the container icon can be expanded to re-display the subprocess within the container.

Subprocess containers can be nested one within another.

A control flow transition cannot cross the boundaries of a container. It can connect to the container as a whole but not to a step within the container.

A message flow transition, which sends a message, can cross the boundaries of a container and connect to a step within the container.