Allied Telesis AT-GS950/24 User Manual page 341

Each MSTI must have a regional root for locating loops in the instance.

MSTIs can share the same regional root or have different roots. A regional
root is determined by the MSTI Bridge Priority value and a bridge's MAC
address.
The regional root of a MSTI must be in the same region as the MSTI.

AT-GS950/24 Web Interface User Guide
341