as it applies to Dark Eldar vehicles:
if the vehicle remains stationary it and any passengers can fire at full effect.
if it moves up to 6" the vehicle and passengers can fire as if they moved (so heavy weapons must snapfire, salvo weapons use the reduced profile).
if it moves more than 6" and up to 12" the vehicle can fire two weapons at full effect and snap fire the rest. Passengers can only snap fire.
Deep striking vehicles count as having moved 6" on the turn they arrive
any questions?