So units in transports behave a few ways.
First, those embarked into a transport don't count as separate drops. So 10 Nobz, Pain Boy, and Weirdboy count as 1 deployment when you deploy the Transport.
Those in vehicles are not in reserves, they are set aside and can shoot from open top vehicles.
Choosing Keyword designation happens during list building. So you decided your Chapter, your Legion, Sept, Dynasty, etc as you build your list. Of you have let's say 2 Captains, one White Scars and one Blood Ravens, then you would specify that during the list creation