Hi Priya,

See by communal rank they make it to do the counselling easier for reserved category student and give them some distinct rank which could be used to allot the college. The rank which is best for you is used in alloting institute automatically. Overall rank is just rank that is totally merit and performance based.

Good Luck !! 

Hello Mohana Priya.

Well usually the candidates get admission based on their general rank and belonging to some community gives the advantage of reserved seats. Other than that, the community rank doesn't really have a solid purpose. Your community rank specifies your position amongst the candidates of the same community. So basically, you will get more chances of getting those reserved seats while getting admission than the candidates of the same community with lesser rank than yours.

Hope that helped.