The Self-service assignment method supports the use of a process data. This process data contains a comma-separated list of usernames.
Those usernames have to be member of the action’s participant.
If a user specified in a process data doesn’t belong to the participant of the action, the action will behave as if the process data was empty: no assignment error will be raised, and all participant members are notified.
In the previous case, if you use the Automatic to the person assignment method, then the action is not assigned and an assignment exception is triggered. You should then set a manual action to be triggered by the assignment error.