Hi Ravindra

Actually mock seat allotment is a type of allotment which is not permanent and it is done to give u a brief idea about that how this year trends of rankings have changed and according to your preference which college are you getting. The difference between mock allotment and true allotment is after mock allotment you an change your preference but once the original allotment starts in corresponding rounds you can't change your allotment. In brief it's just a drill of procedures of original allotment. Mock allotment though are not perfect but gives some ideas. 

Hopefully my answer helped.