Hello Aspirant,
See if you are seeking admission to any engineering courses in any of the NITs or IIITs then you should mandatorily appear for JOSAA or CSAB counselling and during the document verification you have to produce your caste certificate, now the caste certificate can be either of state level or central level but the format should be similar to that of available on the official website of JOSAA, for which link is given below :-
https://josaa.nic.in/WebInfo/Page/Page?PageId=1&LangId=P#
If you don't have your reserved category certificate for your respective category in the above format right now then you can submit the undertaking in the JEE-Main 2021 Application which declares that you will submit the original and valid caste certificate later on at the time of admission.
You can download the undertaking from the official portal of NTA, for which link is also given below :-
https://jeemain.nta.nic.in/webinfo2021/File/GetFile?FileId=5&LangId=P
When you visit on the above link you will find a PDF and on page no.9 there is undertaking format which you have to download and after filling it you have to upload it on the place of category certificate in the JEE-Main 2021 Application form.
I hope this information helps you.
Good Luck!!
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