You can set only one activity per view at a time, and all checkouts in your view are associated with the currently set activity until you unset the activity or set another one.
In a UCM environment, there are several locations from which to find and set activities:
In a project that is enabled for the UCM integration with Rational ClearQuest, before you use cleartool setactivity, a UCM activity object must exist in the stream, and you must know the activity record ID.