Dear,

All India Rank(AIR) is the rank alloted to you when compared to all students in India including general candidates and all category candidates. Whereas category rank is the rank alloted to you when compared to only your category students. Say your OBC category rank is 45 then you are 45th based your marks as compared to all OBC candidates. 

When you are a category student you get admission based on your category rank only.

Hope this helps!