Allied Telesis AT-S63 - Manual (Page 239)

AT-S63 Management Software Features Guide
Section V: Spanning Tree Protocols 239
VLAN and MSTI Associations
Part of the task to configuring MSTP involves assigning VLANs to
spanning tree instances. The mapping of VLANs to MSTIs is called
associations. A VLAN, either port-based or tagged, can belong to only one
instance at a time, but an instance can contain any number of VLANs.