I wrote this as an answer to why someone who tops the UG entrance may not “top” the PG entrance too. The language of the question seemed a bit vague, so I’ve interpreted it my own way.
Part 1:
Top rank is a vague term with respect to the PG exam.
Firstly because, a person needs only a rank that is good enough to get his desired branch. The rank required for a particular branch will vary from session to session (depending on the branch choice of people around).
Secondly. Let’s say, for one session, Aiims Delhi decides to keep 0 seats in UR dermatology (which is a very common phenomenon), even rank 1 will not get dermatology if that person wants only Dermatology. At that point, rank would become meaningless for that person, even though the world would be celebrating his/her success. I believe this session, Aiims Bhubaneswar has kept no seats in Dermatology for UR candidates.
In the UG entrance however, everyone with a rank within 1–40 or 1–50, depending on the number of seats will get into the same place. There’s no distinction based on rank.