'Activities' in BCS- how are they generated?

Hi gurus,

Activities in BCS (as per the activity sequence) are they part of the transactional data being generated at posting level 30? If yes how does the system come to know that this is the activity which needs to be executed. For example if there has been a partial divestiture how will the system come to know that this activity has occured and how the corresponding activity for partil divestiture (10) will be identified so that it can be written to the data.

The activity numbers- are they system genererated or we have to supply the same as part of reported data of AFSD?

Pleaes advise.

BR,

SK