They have not mentioned anything related to that as only below criteria is mentioned -

The JAM 2017 appearing candidates should have secured at least 55% marks in aggregate of the subjects including languages and subsidiaries of all the years combined. Those however belonging to SC/ST and PwD categories need to secure at least 50% marks in aggregate of the subjects.
Candidates need to note that the equivalence of marks in case of grades in letter/CGPA (and not in percentage of marks) will be decided by the JAM 2017 participating institutes.
As per the eligibility criteria of JAM 2017, candidates will need to submit their proof of qualifying the degree examinations by September 30, 2017 to the admitting institute, failing to do so will lead to cancellation of candidature.

For more information, you can go through below article -

IIT - JAM Eligibility Criteria


Good Luck!!!