Dear aspirant,

the percentile score indicates what percentage of the total number of candidates appearing for the exam have scored EQUAL TO OR LESS THAN the score you secured.

This means, in each session, the marks secured by the candidate (say, out of 360 in the case of JEE Main Paper 1) is converted into a scale ranging from 100 to 0. The topper for each session will, thus, secure a percentile score of 100.

Percentile Score of a Candidate = 100 x (Number of candidates who secured a raw score (or actual score) EQUAL TO OR LESS than the candidate) / (Total number of candidates who appeared in that session)

For more details check the below mentioned link,

https://engineering.careers360.com/articles/jee-main-marks-vs-percentile

hope this helps.