The dungeon items screen allows you to add and remove furnishings to a dungeon room.
The list of furnishings available to a room is largely room specific although most of the dungeon rooms support the addition of stairs and sell boxes.
The item screen lists all of the items that can go in the selected room the different colors used to show an item indicate the following:
You are free to have all of the items listed on the furnishing screen in your room but only one of each individual item is allowed.
Unlike ordinary building furnishing you do not actually need to have the matching item in your inventory dungeon masters can add any item.
Removing a placed room item removes it from the game and does not put it back into your inventory.