Yeah, that's the kicker right there. This means that they are one whole unit until they are called from reserves. Which is a shame, because that means the unit must be separated from their dedicated transport if you want to be able to combat squad them when they enter the board.
For example:
You have a 10 man Space Marine Tactical Squad and a dedicated transport Rhino. If you reserve both the 10 man squad and the Rhino, and declare that the squad is embarked in the Rhino, when you come in from reserves, you have automatically denied yourself the ability to use Combat Squads. The unit is embarked on the vehicle, and you cannot have 2 units in one vehicle.
The only way to be able to combat squad is if you Do Not embark in reserves. However, at this point, the rhino is a separate unit for reserve rolls, and you may not have your transport when you enter (or the opposite, may not have a unit to embark when entering from reserves).
I think this is a really dumb way to play, and defeats the purpose of Combat Squads: flexibility.
This is even worse for Razorbacks, and 10 Man Terminator squads with Land Raiders.
Then think about the restriction to movement. Assuming you managed to bring in both the squad and the transport in the same reserve roll: Blood Angel 10 Man Tactical squad and Razorback enters the board. The 10 man squad combat squads.
1) You cannot use your 18" move for the razorback if you want to embark half the squad.
2) You cannot use your 12" move if you want to embark half the squad.
So, now you have lost some of your movement, and, of course, you had to come from reserves, so you have lost turns.