Allied Telesis AT-S63 - Manual (Page 284)

Chapter 17: MSTP
284 Section II: Advanced Features
that you maintain this number, only that each bridge in a region have
the same number.
The bridges of a particular region must also have the same VLANs. The
names of the VLANs and the VIDs must be same on all bridges of a
region.
Finally, the VLANs in the bridges must be associated to the same MSTIs.
If any of the above information is different on two bridges, MSTP does
consider the bridges as residing in different regions.