A Response Center is the room or suite where a pair or trio of Agents lives and receives missions.
Response Centers in general are made of Generic Surface, and contain a console and a bathroom. Any other contents are up to the Agents that inhabit them, and indeed many Agents have been known to completely remodel their Response Centers. However, mission souvenirs are the most common decoration for Response Center walls, especially body parts from Cute Animal Friends and Sue weapons.
Beds, chairs, cushions, and other items used for reclining are often never used in Response Centers, as it is common knowledge that as soon as one get comfortable in one, the console will beep.
RC Numbers
The list of Response Centers and who inhabits them can be found here.