This read-only value is the slot number identifying which blade forms the protection-to-primary endpoint of the packet ring, reading 0 when the ring has no protection active. It works together with its primary-to-protection counterpart to let an admin fully map the ring's current topology during a fault investigation. For example, an admin cross-references csOrionChassisPacketRingProtToPrimEndpoint with csOrionChassisPacketRingPrimToProtEndpoint to identify exactly which two slots currently bound the protection segment of the ring.