Objective
Collections are symbolic containers used to manage sets of objects or activities seen as identical. They may be associated with physical containers yet keep their identity; they come with standard attributes and operations but may be managed as business objects if they are characterized by specific collective features.
Collections and Connectors
Collections can be structural (elements are identified by owners) or functional (elements are identified on their own but can only be accessed through owners).
Applied to activities, collections are used to describe repeated actions and iterations.
Contrary to other structured representations, collections come with a generic (i.e independent of domain semantics) access mechanism and therefore can be used to reference sets of objects across domains.