Group Geofence clarification

Hello,
I am looking for clarification of this statement in the docs around geofences and groups :

If a geofence is linked to a group, it means that the system will monitor all devices in that group entering and exiting the geofence.

By saying "all devices" does this mean that all devices must enter/exit the geofence in order to trigger the event?

Or does it mean thst all devices are evaluated but trigger individually?

Thanks for the clarification.

Individually!

So if I link a geofence to the group and add a few GPS devices to the group, Then the geofence ID will show as NULL in the database if I fetch the list of devices.