what is the reason for a sublot in pharmaceutical production
Could be many. An example would be if a batch of, say filled vials, cannot be autoclaved/freezedried/etc in one load. Then the batch is split into several autoclave loads, each of which is assigned a sub-batch or sublot number to be able to identify which vial was in which autoclave run.
Its actually division of main batch into small lots in order to carry out any further operation as per the capacity of the equipment,
for example coating of 1 batch in 3 lots (sub lots)
or its as per market requirement in packaging.