This is the mask paired with cienaCesAclRuleDscp, letting a rule match a range or group of DSCP values via bit pattern rather than one single code point, and notably the MIB's own description oddly labels it an 'outer' DSCP mask despite DSCP being an IP-header field rather than a VLAN-tag field like the true outer PCP/VID fields elsewhere in this table, likely a copy-paste artifact in the source MIB text. An admin would use a partial mask here to group several related DSCP values under a single rule rather than writing one rule per exact value. For example, a mask designed to match the top 3 bits of the DSCP field regardless of the bottom 3 could group all DSCP values sharing the same class selector into one rule, covering several related code points at once.