The Den is a carrier and is the home base of the JTF Wolverines. The carrier is rather theoretical and does not pertain to the the setting of the campaign. The interior of the ship is only seen in the mission team cutscene while Commander James Rhomer is introducing you to the whole setup of the team.
The interior of the carrier is similar to Retribution, but instead of the SATO logo painted on the wall, it is replaced with the JTF Wolverines logo. It would appear the ship is thus an Admiral-class carrier.