This carries the table index of the pseudowire that is currently active, actually carrying traffic, within a protected pair, as referenced by the notification, letting the recipient identify exactly which PW table entry is now handling the service. This is the active-side counterpart to the primary-PW index fields, and the two won't always match: if a protection switchover has occurred, the active PW index will point to what was formerly the backup rather than the primary. Seeing the active index diverge from the primary PW's own index in a trap is itself the signal that a failover has taken place.