This read-only object is the reference number, 1 through 20, for one specific licensed software option enabled on this node, since a node may have up to 20 separately licensed options active at once and each needs its own table entry. An admin uses this index purely to distinguish which entry in the Codex software-key table corresponds to which licensed feature. For example, entry number 5 might correspond to the frame relay feature license, distinct from entry 1 covering MX25 support.