This is the serial number, from 1 to 65535, that identifies a particular SAN aggregation group and serves as the index into the aggregation group table. Admins use it to reference a specific aggregation group when configuring or monitoring its members, and it's also included in the notification objects for group speed-change traps. For example, an admin might create aggregation group number 3 to bundle several SAN links feeding a particular storage array.