This model is a side loading recessed electrical box.  It is as small as possible when utilizing an old work electrical box (in the USA).  I utilized it when I had a chimney behind drywall and needed to fit this box in under a 3" (73mm) space from the face of the drywall to the masonry.  This is designed to have the new work box mounted to the side with the flip out tabs that rest on posts and has guides to keep the electrical box in place.  There is a seperate cover to be put in place after you install the main housing so that you can still service the electrical if needed.