Since Activity is a subclass of ManagedObject, it has an owner property, which is an Organization, which can be a Project. If the owner is a Project (the usual case), the Activity occurs in the context of the Mission, a special subclass of Activity that is defined as the top level Activity of a Project, so all other Activity instances that occur within the Mission are sub_activities either of the Mission itself or of another Activity within the scope of the Mission Activities.
In general, an Activity is performed by an item, which is designated by the
of_system property, which can be populated by an instance of either Acu or
ProjectSystemUsage. All instances of Activity must have their of_system
properties populated by a usage that occurs in their owning Project, except
for instances of the Mission subclass of Activity, for which of_system is
not populated since the Mission is not an activity of a specific system but of
all project systems.
Any Activity instance can be decomposed into a set of sequential subsidiary
Activity instances. The sub_activity_of property refers back to the parent
activity and is the inverse property of sub_activities, which points to the
list of children of the parent activity.
An Activity has parameters start, end, and duration.