My assumption is, that “fully modded” refers to a state, where the game is like an entire new release (new game or Remaster in example). Partially modded could be in example just changing a few things, without altering the entire game, maybe new weapons or a new character or a region of the map changed. There is no clear definition what these vague terms mean. I just provided a vague answer to a vague question about a vague topic. :D