Although the answers below, clarifies most of the part for the question.
I would like to tell that sliding round after all three rounds of counselling is same as last round of counselling.
Any candidate who has been alloted a seat till the 3rd round of the counselling AND who hasn’t freezed his or her seat yet are eligible for sliding round. Further, similarly the way in normal rounds of counselling, if a student don’t get an upgrade of seat in sliding, the seat from last round (that’s 3rd here) remains retained. Now if you don’t freeze your seat whether upgraded seat from sliding or upgraded from 3rd, then these seats go to the spot round or open house counselling. And spot round is altogether a different scenario here. So it is advisable to freeze a seat after sliding round.