This tracks how much of the priority-5 bandwidth allocation on this link is currently committed to tunnels signaled at that level. An admin auditing where a link's overall reservable capacity is going would use this figure, alongside the equivalent values at other priorities, to build a full picture of demand across all eight priority tiers. A near-zero Reserved Priority 5 value alongside heavy usage elsewhere would indicate this particular lower-priority tier simply isn't being used much on this link.