Hello keerthana. Register number is nothing but your roll number of class 12th. Whereas the subject code is the one which is adjacent to each subject in the 12th hall ticket. Do not confuse both. I have attached a link which directs you to Directorate of Technical Education called as DOTE who is responsible for Tamil Nadu Engineering Admission ( TNEA ) online counselling.

http://www.tndte.gov.in/site/

Contact the DOTE help desk for filling TNEA application. They will clarify all your doubts regarding TNEA online counselling. Be notified about their working hours. They are function weekdays from 10 am to 5.40 pm. On weekends, Saturday - 10 am to 3.55 pm. Sundays are holidays. Their contact number is 044 2235 1014 (https://www.google.com/search?q=dote&rlz=1C1OKWM_enIN903IN903&oq=dote&aqs=chrome..69i57j46j69i59j0l5.1590j0j7&sourceid=chrome&ie=UTF-8#) .

Hope this was helpful. Thank you. Stay Safe.