On Beitzah 30b the gemara cites to a mishna which states that wood cannot be removed from a sukkah on yom tov. Initially, the gemara distinguishes between the wood which is part of the skach and wood which is part of the walls, but within the context of a yom tov other than Sukkos. However, later the gemara begins a discussion as to whether wood can be removed from the sukkah on the Sukkos holiday.
Tosafos (d"h Aval) asks whether the gemara's discussion is on a Rabbinic or Biblical level. Tosafos notes that the gemara implies that the wood cannot be removed because it is muktzeh, yet previously the gemara had compared the sukkah to the shalmei chagiga, which would imply that the prohibition is Biblical in nature.
The Sefer Sha'arim Hamitzuyanim B'Halacha offers numerous answers to this question. He first mentions that Rabbeinu Tam (as mentioned in Tosafos) states that the Biblical prohibition only applies to the two and a fraction walls which are required in order to construct a kosher sukkah. He also notes that Rosh differentiates between the skach which is Biblical in nature and the walls themselves.
However, he also brings an opinion which interprets the Rosh as stating that it is only the minimum size of the sukkah (7 x 7) which is Biblically prohibited from use. This presents a problem (which he does not answer) in that there is no concept of bereirah for Biblical prohibitions, so how does one identify the permitted and forbidden portions of the sukkah?
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