An encyclopedia can have more than one complete activity model. An activity model is defined to consist of a single context activity diagram, A-0, with its single A0 activity (and supporting External activities Ext Ain and Ext Aout) and, subsequently, decomposed to whatever depth is required.

Normal business processing rules for activity modeling do not allow the same activity to be used multiple times within a single activity model. This means you can not have, for example, 1.3 Plan Action, and somewhere else in the same activity model have 1.2.1 Plan Action. You need to rework the activity model so that there is only a single occurrence of Plan Action somewhere.

The following image shows an invalid Activity Model in which an activity is used twice.

With multiple activity model trees, it is possible for a single activity symbol to appear in each activity model (one time). So that, in Activity model Engage, you might have 1.3 Plan Action and in the Activity model Perform you might have 2 Plan Action. In each instance, there is a single definition of Plan Action that both symbols are associated with. If any property of the activity definition for Plan Action changes, then all occurrences of the symbol Plan Action change accordingly. Within System Architect, these models must be partitioned into different encyclopedias so that the leaf status of 1.3 Plan Action does not conflict with the parent status of the 2 Plan Action.