Dear,

One is allowed to fill in his/her choices by 28th June and mock seat allotment is scheduled on 27th June which means that on 27th Josaa the seat allotment authority will display the mock seats allotted to the candidates. These are mock and not final seats. It is to make you know that what seat you would get based on your college preferences so you can change them later before the final seat is alloted. It is nice way predict what seat you will be alloted so you can make certain changes.

Hope this helps! Good luck!