Elections
(1) The President, by public notification (consulting the Governor for a State), specifies which castes/races/tribes (or parts/groups within them) are deemed "Scheduled Castes" for that State/UT. (2) Parliament may by law add to or remove from this list, but no other authority can vary a notification once issued.
This establishes exactly how the official "Scheduled Castes" list is created and maintained - a formal presidential notification process, with only Parliament (not the President alone, nor any state) having the power to subsequently modify that list.
This is why whether a particular caste is officially recognised as a "Scheduled Caste" (and therefore eligible for reservation benefits) in a given state depends on a specific presidential notification for that state, which can only be changed by an Act of Parliament, not by executive order alone.