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"Specially modified thunderhawk transporters" is how they were listed as being recovered in an old Imperial Armour book (on Astartes vehicles).

No further elucidation on the types of "special" modifications was given.

It always just makes sense to me that Space Marines will have specialized transporation vehicles intended for use only after a warzone is secured.

They will have to retrieve heavy equipments and rapidly redeploy/recover for them to serve as advanced forces they are supposed to be. Any heavy equipment used in fortifications, combat vehicles, supplies for said vehicles, supplies for forward bases and their combatants, heavy ordances and munitions, rations and medical supplies etc. all have to be brought from their Battle Barges to the ground and back, and it's the fastest if they have a vehicle specifically designed to ferry huge amount of wares to a secured supply depot for redistribution on the ground, so these vehicles don't have to carry armaments of their own.

Given it's the Imperium, it's certainly gonna be based on existing vessels, with any strippable weapons and ammo storage compartments removed, as far as the AdMechs would turn a blind eye on. But I'd imagine every chapter fleet would maintain a sizeable number of these vessels, perhaps even retrofitting older models that's been too "lamed" for combat duty for this specific purpose.