In Kerala, the Secondary and Higher secondary board are based on 9 based scoring system. A person who gets a 9 score gets the equivalent grade of A+. Now the score 9 is provided to those who scores 90 to 100 % in aggregate.

For example, if you have 6 papers each of 100 marks then the total score is 600. Now for A+ grade you need get 90% of 600 i.e., 540 marks at least and 600 marks at most.

I hope this helps. Wish you a great day!