A (user) Activity is a single action performed by a user to fulfill a process step. I.e. normally more than one Activity is required to perform a process step. It is the lowest level of granularity that is foreseen within this structure model.
Activity level of business process modeling is typically used when user interface design and description is necessary.
1. Enter customer Name
2. Enter material quantity
1. An activity can only be related to one business object.
2. Lowest level of granularity
3. All activities done within one process step by one role could possibly be basis for designing an application supporting the step.
Illustration in ARIS