When is a mod more than a mod?

There are games that exist as a platform. Tabletop Simulator is a perfect example - The purpose of the game is to make all of a common mode of play inside a computer - anything you usually play with a tabletop. It's a bit clunky and odd, but it certainly serves its purpose.

On the other end of the spectrum we have console games. They are solid disks of software that never change and are exactly as they were when they shipped. Pick any number of games and they would go here.

In the middle we have a number of games that can be changed, particularly on PCs. We can alter the music or sound, in some games we can dig inside and change how the game itself works. We call this modding, and we change some games so much they don't resemble themselves anymore.

However... there exists something inbetween, not quite game and not quite mod. These can only exist in certain games, like Minecraft or Garry's Mod, and they can exist alongside other things. They are little games unto themselves that could easily spin off into their own creation. Some of them even span multiple games.

There appears to be a substantial difference between something that changes the game and something that uses the game as a platform, even if they both exist side-by-side. I'd like to clarify this. What do we call these?