Hello,
In case you are pass in all 6 subjects in class 12th board exams then there will be a comparison between 5th and 6th subject(except pcbe or pcme).The subject in which you got maximum marks(from 5th and 6th) will be used to calculate your class 12th board exam percentage.
If you are fail in one of the subject then the 6th subject's marks will be included in order to declare you pass.
Good luck.