Activities are the fundamental concept for defining processes.  Activities define the breakdown as well as flow of work.  In other words, Activities can be nested into each other defining a breakdown structure of work or they can define predecessor relationships to other Activities defining a flow presented in Activity diagrams.  Activities can also contain references to Task, Roles, and Work Products called Descriptor.  Activities as well as Descriptors relate to timelines by allowing their instances to define start and/or end dates.  Further, they specify information relevant to the instantiation of work in project such as if an Activity shall be performed several times and if so if they can be performed in parallel (hasMultipleOccurrences attribute) or one after other (isRepeatable attribute).  Activities and Task Descriptors can also be event driven or describing ongoing work that does not have a fixed start and end time.

UMA defines several special Activities that allow expressing processes with terms many users are familiar with.  For example, Phase or Iteration are just special Activities for which specific attribute values have been set with predefined values. A process such as a Capability Pattern or Delivery Process is also nothing else than just a special Activity that contains additional documentation on why and how to use the process. Hence, because Activities could be nested into each other, so can processes.