Hello Jitendra,

it is the system to select candiidates for best colleges used by JEE System.

1. from +2 they consider major papers like physics,chemistry,mathematics, a language paper like english and another paper except above 4.

2.if any candidate have taken two language papers,then the paper with higher marks will be considered.

3.apart from PCM, if there is any 5th subject in which the candidate has secures more compared to other then that subject is taken into account.

4. If there is any subject whose total marks is not 100 then that is converted to 100 and then calculated to make aggregate of 500.

for more details you can go through the website given below:-

https://engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission

Hope it Helped!