Procedural room contents can have a RoomCenterPattern.
This Room center is just an Actor which will be spawned at the center of the room and which you can use to create custom patterns inside your rooms.
First example : the Begin Room and End Room have a room center which spawn a gate inside the room (see RoomCenter_Transition_01).
Second example : RoomCenter_01 spawn several traps and a chest at the center of the room.
If you want a room to have a room center you need to setup the Data Table to tell the room content which room center it will spawn :
The MinOffset parameter is the number of cells to add to the center to determine if a room center has enough space to be spawned.
You also have a MinRequiredSizeX and a MinRequiredSizeY params inside the room center.
Those parameters need to be correctly setup otherwise the room content won't spawn.