They are different ships. The Space Marines have many different 'interface craft' - they've never shown up in
40k, but used to exist in epic:armageddon.
Thunderhawk Transporter (rhino carrier)
Carries 2 'rhino chassis' or one 'land raider chassis' instead of the normal troop complement
Landing Craft (massed armour landing barge)
Carries six rhino chassis vehicles AND twice the troop capacity of a thunderhawk. Not as maneuverable as a Thawk, and no good in a gunship role, but better armed with defensive turrets to protect itself whilst landed - designed to provide a 'drop in' strongpoint.
A Strike Cruiser generally carries 3 Thunderhawks and 1-2 of these things to bring down attached rhinos, razorbacks, predators, etc.