It’s the shape of a shirt, skirt, hair, glasses, furniture, shoes, etc.
Depending on the game, they may be tagged as #TS4CC or #TS3CC on various places like Twitter or Tumblr.
Here is the most commonly used terminology when dealing with The Sims 4 custom content.
If you’re new to the Sims franchise and you’ve looked into custom content, you have probably stumbled upon some specific terminology regarding custom content.