What is a "Title Split"?

My current understanding of a Title Split is this:
A Title Split is used when a game adds achievements as part of a DLC, but some of those achievements can be earned in the base game (A.K.A. you can earn achievements for a DLC you don't even own). Anything that can be earned with ONLY the base game are then removed from the DLC and are instead treated as part of the base game (at least for the sake of 100% completing a game).

That said, I have 2 questions:
1. Is my understanding of this functionality correct?
2. How do we request one? (One of the games I've played will not let me mark it as complete due to starting DLC I don't own)

The base game is the achievements at launch. Anything added after launch is put into a title update or DLC pack separate from the base game.

Make a thread in the games forums, or submit information in the dropdown on the games page