This read-create value is the mask applied to aclRuleIpTosBits, determining which specific bits of a packet's TOS field this ACL rule actually checks versus ignores. An admin sets this mask to isolate only the specific TOS bits relevant to the classification, ignoring the rest of the field. For example, a narrow aclRuleIpTosMask that isolates only the throughput bit lets this rule match on that one TOS characteristic regardless of the other TOS bits' values.