Hello Shaswat, 

After the process of document verification, one need to report to the the alloted institute or university and confirm the seat allotment by fulfilling the formalities like filling some forms and all, then you are all good to go to join the college from the very first day.

And yes, once you freeze the alloted college option after document verification, then your admission gets confirmed in the last alloted institute. 

Hope it helped you..