The ARC Carrier is an intermission level in The Ancient Gods - Part One, the first DLC of Doom Eternal. It appears in cutscenes in each level but also has its own short playable portion between UAC Atlantica Facility and The Blood Swamps in which you get to run along an extending walkway and jump into a Teleporter.
Overview[]
The Armoured Response Coaltition was designed by Dr. Samuel Hayden after global defense systems began to fail, and while it functions as an aircraft carrier and is capable of long-range bombardment its primary function is as a source of intel for the ARC.
the Doom Slayer used this carrier for only one purpose, to choose which location he should go to next. In ancient gods part one, the Slayer with the help of The Intern travelled to different locations such as UAC Atlantica Facility, and on the advice of The Seraphim, he travelled to Blood Swamps and after that, The Holt to resurrect the Dark Lord so Doom Slayer may permanently kill him and end the existence of all demons.
In Ancient gods - part two, the carrier has not been seen at all. Though it might be implied that the slayer did use it off screen.