This is the mask paired with cienaCesAclRuleOuterPcp, letting a rule match a set of PCP values via bit pattern rather than a single exact priority level, though given PCP is only 3 bits the practical range of useful masks is fairly limited compared to the VLAN or IP masks elsewhere in this MIB. An admin might use a partial mask here to match a small group of related priority levels together in a single rule rather than writing one rule per priority value. For example, a mask designed to match both PCP values 6 and 7 together would let an admin write a single rule flagging both of the top two priority classes rather than needing separate rules for each.