This enumerated/bitmask value lists which IPv4 header fields (source/destination address, protocol, etc.) this flow-monitoring record definition captures as key fields when building flow records. Because flow-monitoring records are only as useful as the fields chosen to key on, an admin sets this on this flow-monitoring record definition to determine whether exported flow data can, say, distinguish traffic by source IP versus lumping it all together — directly shaping what the downstream NetFlow collector can report on. A value including 'srcAddr' and 'dstAddr' would mean this flow-monitoring record definition keys IPv4 flow records by source and destination address.