Activity modeling in SysML emphasizes the inputs and outputs, sequence, and conditions for coordinating other behaviors, instead of who owns those behaviors.
When you create an activity, you automatically create an activity diagram. An activity can be modeled under use cases. The use cases activities can be run and simulated the same as class activities. An activity can also be modeled under an operation. Invoking the operation will result in executing its activity. The activity implementation can be visualized.
You can also model and simulate time events and specify pins for send and accept event actions. Use the auto-synchronization mechanism for quick and safe mapping.
Rational Rhapsody adds both a new activity
and, under the activity in the browser, a new activity diagram to
the model, and opens the blank diagram in the drawing area. You can
change the default activity name, but not the diagram name. It is
useful to immediately show the activity frame using the Show/Hide
Activity Frame option. 
SysML activities specify the following information:
See Creating Harmony-SE projects for more information about systems engineering activity modeling in Rational Rhapsody.