There is no specific closing cutoff for the mock-round allocation. Though based on the previous year's data, the closing rank is supposed to be in the range of 1500-2000. It can increase but it is less likely to drop below 2000.
The mock seat allocation is done on the basis of options filled in by the particular candidates within the time limit. It is however meant to provide each student an overview about the status of the option filled in by them for their personally opted IIT course, as per record.
It is worth keeping another notice that the choice filled by an individual can only be modified as per data until the set time limit.