This read-only BITS value reports which optional Ethernet OAM functions, unidirectional support, loopback support, event support, and others in the full bit list, this local interface actually implements, per its description, since basic discovery is mandatory but these functional groups can be supported independently. These bits are what get communicated to the peer during discovery and determine which OAM capabilities, like remote loopback testing, are actually usable on this link, regardless of what the operator might try to configure. An engineer trying to run a remote loopback test who finds it doesn't work would check this bitmask first to confirm loopbackSupport is actually set for this interface before suspecting a configuration or peer issue.