Some transport vehicles are directly assigned to a particular unit. Normally, a unit entry in a codex will include a transport option allowing a vehicle to be selected along with the unit. Other transport vehicles occupy a force organisation chart slot (for example Eldar Falcons) and can be used to provide ad hoc transportation to any unit that can embark on it.
No more than one unit can ever be embarked on a single transport.
Dedicated transports may only be used to transport the squad they are selected with, plus any independent characters who have joined the squad. An independent character may disembark and leave the squad that owns the transport embarked, but if the squad disembarks then any independent characters must do so as well.