Hey
Category A refers to the seats allotted on merit basis. (General Fee Structure applicable)
Category B refers to the seats allotted by charging higher fees than the General Fee Structure

To know about complete fee structure, visit: http://www.srmuniv.ac.in/admission-india/fee-details/engineering_and_technology

Category A means the seats which are officially announced from various department of the college and which are filled on the basis merit whereas B-Category seats are the management seats as if you opt for category B seats then the fee would be higher than Category - A.

Good Luck!!!