sliding round is done after all three rounds in ipu .usually it is done to fill the vacant seats if it is arised after these rounds, else the students who have been alloted seats in previous counselling can also have chance to upgrade there seats to their higher priority, so sliding round is meant for only those who have not been alloted any seats in early counselling slots or even if seats has been alloted they have not freezed there seats yet.
hope it helps!!