Dear Rabbi,

At the beginning of maseches sukkah, the Gemara asks why the language in sukkah (psual) is different to the language used in mavoi (yemaet). Both answers of the gemara seem to say that the loshon of psula cannot be used when referring to a derabanan rule. Rashi seems to explain this as a logistic reason, we weren't given any prior measurements so we can't refer to a prior "law" to say it doesn't conform with that law. However, the gemara on daf gimmel, when referring to the dispute between Beis Shammai and Beis Hillel about a sukkah that is not large enough for rosho rubbo and shulchano, Beis Shammais says poslim. I am assuming that Beis Shammai only posuls the sukkah due to a derabanan that one may be drawn after the table, but nevertheless, they use the term poslim. This seems to contradict the rule that gemara earlier established, that the word posul wouldn't apply to derabanans.

Thank you,

Avrohom, Israel