The Gemara on our daf discusses the concept of dofen akuma – a curved wall. In cases where the skhakh – roofing – does not reach all the way to the walls of the sukka, if the distance between the walls and the skhakh is less than four amot, we apply the rule of dofen akuma and perceive the wall as reaching the skhakh. Two cases that make use of this rule are mentioned in the Mishna: the case of a house whose roof has been breached in the middle and is replaced by skhakh for the holiday, and the case of a courtyard that is surrounded by an akhsadra – a portico or covered area, with an space left in the middle that is used as a sukka.
Several explanations are given to explain the mechanism behind the workings of dofen akuma. Rav Nissim Gaon explains that halakha simply perceives the wall as moving to a position where it abuts the skhakh. Most rishonim (Rashi, the Me’iri and the Ran) understand that we consider the roofed-in area to be part of the wall, recognizing that it is a part of the wall that runs horizontally to the point that it reaches the skhakh, rather than vertically as we usually expect walls to be. The Peri Megadim suggests that halakha perceives the wall as rising at an angle to meet the skhakh.
One of the practical differences that arises from this argument is how to rule in a case where the skhakh is higher than the top of the walls. In that case, according to the accepted opinion that the ceiling is seen as a horizontal wall that reaches the skhakh, here it does not reach the skhakh and it is likely that we will not be able to apply the rule of dofen akuma.