7th ed. Rulebook, page 120:
'Sometimes a unit's Army List entry will include a Transport option, allowing a vehicle to be selected together with the unit. These Dedicated Transports do not use up a slot on the Force Organisation Chart (pg. 120), but for all other purposes count as having the same Battlefield Role and Faction (if any) as the unit they were bought for.
For example, a Rhino bought for a Space Marines Tactical Squad (Troops) counts as a unit of Troops, but one bought for a Space Marine Sternguard Veteran Squad (Elites) counts as Elites.
Other vehicles may also have a Transport Capacity, but as they are chosen seperately, they have a Battlefield Role and occupy a Force Organisation Chart slot of their own'
Dedicated Transports are allways an OPTION for a certain unit, to be selected together with said unit.
This is clearly indicated in 5th and 6th edition codicees.
... and a unit entry:Codex: Dark Eldar 5th:
'Certain units have the option of selecting a Dedicated Transport vehicle ... '
Same in 6th ed. entries:'Options:
- The squad may take a Raider or Venom as a Dedicated Transport (see page 91 ... '
Codex: Eldar 6th:
- The unit may take a Wave Serpent as a Dedicated Transport (see page 97 ... '
Again, Dedicated Transports are always a very specific option for a specific unit.