Hello David
You’re on the right track. In MSTP, the “Regional Root Identifier” indeed identifies the root bridge for each instance. It is used to identify the root switch of the region for a specific instance. The bridge ID contains both the priority and the MAC address combined together - Bridge priority + MAC (System ID Extension + MAC in MSTP).
The Bridge Identifier of the local switch is located in the “CIST Bridge Identifier” field. You can see this as a field in the first screenshot you shared. The CIST (Common and Internal Spanning Tree) is the spanning tree that runs in an MST region that interconnects with other MST regions and with other types of spanning tree protocols.
As for the Proposal and Agreement bit being set at the same time, this may happen during the negotiation process of MSTP. The Proposal and Agreement flags are used to establish a rapid agreement on forwarding ports. When a switch sends a BPDU with the Proposal flag set, it is proposing to the downstream switch that it (the sender) should be the root port. The downstream switch, if it agrees, starts sending BPDUs with the Agreement flag set. This is how they communicate and agree on the state of the ports.
I hope this has been helpful!
Laz