It's a variation on the service hatch, but with adapted material properties that make it completely invisible.
I use it
- to hide a ground floor wall in building with more than one floor
- to make an overhanging second floor
- to make passages in a wall without a door frame
- to make two rooms on the second floor, but just one room on the ground floor
I attach the file. When you import it, you won't see a picture, but that's to be expected with invisible objects
Hans