Dear,

In sliding only those candidates can participate who have been allotted a seat in the online rounds but have NOT FREEZED their seat. Such candidates can participate in this round in which they will be upgraded in their choices if the seats are available.

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.

Hope it helps!