This read-write Integer32 defines the protocol subtype value that a CPD filter row on an Ethernet Network Port must match, refining the filter beyond MAC address and Ethertype alone. It matters the same way as the flow-level subtype object: some protocols share MAC and Ethertype but are distinguished only by subtype, such as EFM OAM vs. LACP Marker both using Ethertype 0x8809. For example, to filter specifically EFM OAM frames rather than LACP Marker frames (both sharing Ethertype 0x8809 and a common MAC range), an admin would set cmEthernetNetPortCpdSubtype to the EFM OAM-specific subtype value 0x03.