A more thorough explanation:

Supernumerarii were officials in ancient Rome who were not part of the permitted number. They were advocates who were not registered and not attached to a particular bar.

• John was a supernumerarii lawyer who was not registered with the bar association.
• The emperor appointed several supernumerarii officials to help him govern the empire.

These examples show that supernumerarii were officials who were not part of the permitted number. They were either unregistered or not attached to a particular bar.