Are you sure about this? I wondered about outgoing traffic myself. I think it's passed by default. Otherwise, I'd need a rule to let LAN traffic out to the Internet and I don't. When I wanted to preclude the possibility of crosstalk on disparate LANs (henceforth, LAN means LAN and OPTs), but still let traffic through to the WAN, I had to use an allow rule on the LAN interfaces (plural) that passed all incoming traffic except that bound for the LANs (the allow is necessary since incoming traffic is blocked by default). Blocking outgoing traffic on LAN interfaces with LAN source addresses did not work (but I may have done something wrong).
In answer to thorrr's original question, a rule on the LAN interface that affects packets with LAN source addresses and other-than-LAN destination addresses is an inbound rule (likewise on any interface). The traffic comes from the LAN, is allowed through (or blocked) at the LAN interface by the rule, and then, if passed, sent on the next hop to its destination. Theoretically, it could be passed or blocked at the outgoing interface as well. In this case, a rule on the interface for Network X that affects packets bound for Network X and originating from other-than-Network X is an outbound rule (traffic comes from wherever and must pass through the interface en route to Network X) . For whatever reason, I could never get this to work.
A point of clarification: here, inbound (incoming) means entering [outbound (outgoing) means leaving] the pfSense router/firewall on any interface. Do not confuse this with entering or leaving the local area network(s) in general.