This read-write OCTET STRING is an 8-octet mask applied to the packet's arbitrary field before comparing it to filterField. An admin uses this to ignore irrelevant bits within the 8-byte comparison window, refining a custom payload-based filter to match only the meaningful bits. Setting most of filterMask to zero except a few key bits lets an admin match on a narrow flag or type field embedded within a larger, otherwise-variable byte range.