No,it is not compulsory to join that institute under the preferred branch filled by you i.e. the preference  filled by you in the option form.But you must have to report the institute alloted with fee and other document verification.It is must to report if a seat is alloted.If you wan't report then your seat will be cancelled and you will not be considered for further rounds.During reporting you can use freeze or float option to confirm the seat or to be considered for further rounds.

Hope i am clear.

Hello, No, it's not compulsory to join the seat after the 1st round. There's no such rule which says that student is bound to take admission after 2nd round allotment. Although, some colleges try to force students to take admission, but beware of such colleges. You can always leave your seat.