New work vs Old work, New Construction vs Old construction can be confusing when standing in the electrical aisle at the hardware store. However, it's pretty simple. The differences you can see on the box, but remember, if you're doing a remodel, you want OLD work electrical boxes.
Old work usually has some sort of tab for install into existing walls and drywall
New work means you have access to studs and beams to nail directly to before putting in the drywall.