This read-write Integer32 (0 to 2147483647), immutable once the entry is created, identifies which session set a given session group belongs to, and the MIB notes the same session set cannot be assigned to two different session groups. An admin cares because this linkage is what associates a group with its redundancy set, and the uniqueness rule prevents accidentally sharing one set across conflicting groups. For example, when creating a new session group for redundancy purposes, an operator would set this to the target session set's number exactly once, since it cannot later be changed.