Hello Aspirant,

Your caste certificate is required both the times, for registration of JEE-Main you have to either upload your caste reservation certificate or you have to submit an undertaking which says that you will submit the original and valid caste certificate later on at the time of admission.  So at the time of admission you have to any how produce your caste reservation certificate otherwise you would lose your candidature even. So take care of this thing.

I hope this information helps you.

Good Luck!!