So in the last game I played there was a question about how far a transport can move if it hadn't moved prior to the squad disembarking. We played it as a max of 6" to make it simple (and it didn't really matter to the results of the game since it was the last turn and the vehicle was still in range to shoot whether it moved 12" or 6"). But after looking up the rule after the fact, it appears that a transport can move normally - no restrictions at all - if it hadn't moved prior to the disembark.
The ruling is found on page 500 (of the enhanced iOS version of the rulebook) under Disembarking:
"If the vehicle had not moved before the unit disembarked, the vehicle can then move normally."