This is a 'give me the next free slot' helper object: a management station reads it to learn what index value it should use when creating a new row in the cpwVcMplsOutboundTable, and a value of 0 means the table has no unassigned indexes left to hand out. It exists specifically to avoid race conditions between multiple management stations trying to provision outbound MPLS entries for pseudowires at the same time. If this object returns 0 on a router that's supposed to still have capacity for new outbound tunnel mappings, that's a sign the table has hit its implementation-specific size limit and needs cleanup of stale entries.