Speed limits and some mass rules for a truck-and-trailer talk about the MAM of the vehicle or combination ('sleep'). What does that mean in practice?

1. Only the tractor unit's MAM counts once a trailer is coupled
2. Only the trailer's MAM counts
3. You must use the authorised mass of the whole combination when that is the legal reference
4. You may choose whichever of the two plate figures is lower to get a higher speed