Lineage

Yip Man / Ip Man is known as Grandmaster even though he was not officially given this title through lineage. I prefer to call him Grandmaster because he was one.
No one appointed anyone else Grandmaster of Wing Chun.
Lineage is often used as a marketing gimmick. Yip Man had a large number of students. Not all of them truely understood Wing Chun. But, they can claim to be a student of Grandmaster Yip Man, and their students claim the lineage. Buyer beware.