Activities are commonly used to describe the behavior of use cases. In SysML these are separate model elements: the use case and the activity. The use case is a specification of behavior. Whereas the activity is the definition of the implementation of the behavior. Instead of an activity you can also use interactions (sequence diagrams), state machines or opaque behavior. However I favor the activity. Typically in a SysML model the activitiy is in the namespace of the use case. You...

It is very well unknown and I know only a single modeling tool that explicitly supports it: the SysML activity tree. Although it is very useful and supports common systems engineering practice. What am I talking about? You may know use cases and activities to describe the use case flows. Simply spoken they describe the functions of the system from the user perspective. The following figure shows a use case and the activity of a forest fire detection system (FFDS), The...