Inside the cell, you can choose to spawn either regular Actors, or Room Items.
In the DataTable, RoomItems are labeled as "Item" wherease actors to spawn are labeled as "ItemAsActor"
Those RoomItems are Actors which have additional functions ( GenerateInRoom() and GenerateInPath() ) which let you perform actions during the generation of this actor.
By using those functions, you can modify the look of what is spawned inside the cell and/or add additional spawning behaviors.
You can open the Cave example in the project to see an example of this technique.
I use the RoomItems located under the PlaceableItems/ folder to generate rounded corners for the dungeon.