Gemarah daf 4a: When the gemara gives the first case of the Sukah which is taller than 20 amos and the Itztebah is on the back wall, the gemara calls this wall a "Dofen". Then, in the case where the Itztebah is 0put against the second wall, the gemarah says "if there is a distance of 4 amos from the edge of the itztebah and the farther wall, the sukah is pasul. If you notice, the gemara reffered to the farther wall as a "kotel". In the next case of "Bayis SheNifachas" with sechach on top, the gemarah refers to the walls of the house as a "kotel". And then, in the last case of the Itztebah, the case where tit is in the middle, the gemrah calls the walls of the sukah "dofen" The Rambam writes in Hilchos Sukah Perek 4 Halacha 14 the Rambam also refers to the walls of the sukah as "dofen", "kotel", and towards the end of the halacha he even calls it a "mechitza" which does seem to appear on daf 4a in regards to cases of the itztebah. My question is WHY???
Good question. We have been looking into your question but have not yet come up with any satisfying answers that cover all of the points.
Usually, "Mechitzah" is the general term which refers to a Halachic separation between two Reshuyos. It is usually not used to refer to a wall of a Sukah.
"Dofen" refers to any type of wall, while "Kotel" usually refers to a more permanent wall, such as that of a house.
In the Sugya of Itzteva, it could be that the Gemara uses different terms in order to help us keep our bearings: the wall against which the Itzteva is placed is referred to as the "Dofen," while the wall of reference (to which we measure the distance from the Itzteva) is referred to as the "Kotel." This is done merely as an easy way to remember that we are dealing with two distinct walls. (It could, be also, that it was the normal manner to build a Sukah by building a third wall between two pre-existing, parallel walls of houses on both sides. The "Dofen" is the temporary wall that was built, while the "Kotel" is the wall of the house. This would explain the case of a Sukah less than 10 Tefachim, in which case there is no mention of a Dofen, but yet there is mention of Kotel.)
We will keep looking into this question, and let you know if we come up with anything else.
All the best,
Y. Shaw