Context:
The latest design iteration of event based process event sources handles the external event source similar to camunda event sources in the sense that:

• it needs to be added explicitly to be available
• it can be removed
  For this the API was extended with a event source type with OPT-3254.

AT:

• external event source can be added & removed from an event based process
• there can be at most one external event source present on a particular event based process