It is hard to guess what you mean by this question, but I believe it is related to the Cache Page, where you can see "(537 m ↥)" or (-2 m ↧) next to the coordinates.
This is not related to the premium membership, but it is there because you installed PGC Browser Extension, which is making the cache listings more pretty and useful:
https://project-gc.com/Home/UserScript
It simply estimates the elevation of the cache based on various sources. This elevation is later used all over PGC, for example for statistics.
When you see -2, it is either an error or the coordinates point some place to the sea.