This read-only enum reports which direction of traffic flow, transmit(1), receive(2), or both(3), is currently being mirrored to the probe port, and per the description of ciscoTsPassiveProbeIndex, this actual direction is determined by the combination of index and the monitored port's duplex mode rather than being independently settable here. An admin trying to capture a specific direction of traffic, say only what a server transmits, would need to understand this interaction rather than assuming they can simply set direction like a normal read-write config object. Seeing both(3) confirms the monitored port is in half-duplex, since full-duplex ports only ever expose one direction per probe index.