A unit that begins its Movement phase inside a vehicle can get out either before or after the vehicle has moved. When the unit disembarks, the models are deployed at least partially within 2" of one of the vehicle's access points. If the vehicle has already moved then the passengers cannot move other than to deploy, and they cannot disembark if the vehicle moved more than 12".
This 12" move limit includes all bonus movement from special engines, frenzied driving, using roads and so on (and that does mean all bonus movement, with no exceptions. This over-rules anything previously published in a codex or FAQ). If the vehicle has not yet moved then the infantry may move as normal after disembarking, and the vehicle can then move off separately at any speed.
Troops on board a vehicle may not disembark in the Assault phase and may never disembark within 1" of an enemy models. If compelled to disembark, because of the vehicle being destroyed, for example, any models that cannot disembark because the access points to the vehicle are blocked by enemy models become casualties.
After disembarking, models may shoot but may not assault unless the vehicle they disembarked from is open topped.
Diagram: Disembarking Before Vehicle Moves. Troops disembark from a vehicle in a fan of arrows before the vehicle itself moves. Caption: "The vehicle did not move before the troops disembarked so both the vehicle and the troops can move as normal after the unit has deployed."
Diagram: Disembarking After Vehicle Moves. The vehicle moves first (arrow), then troops disembark within 2" of an access point. Caption: "As the vehicle moved before the troops disembarked they can only be deployed if the vehicle moved less than 12". The vehicle cannot be moved any further this turn."
Designer's Note: This particular addition should attract a lot of interest. Players will have to rethink their transport vehicle tactics quite a bit to cope with it. Compared to previous editions of the rules, transport vehicles have enjoyed a golden era, perhaps to the extent that the pendulum has swung too far. This rule is an attempt to redress matters a little.