Hi Justin,
I’ve found out that this flag covers scenarios where the PBX doesn’t know how to route the call.
But in reality, if a ring group is being as a “link in a chain” such as Auto Attendant > Main Ring Group > Overflow Ring Group, if all the members in the main ring group are set to DND, then instead of following the call forwarding rules to send calls to the overflow ring group, the calls stop at the main ring group voicemail.