פירוש על אהלות 6:4

כותל שבין שני בתים- and the wall that forms partition/separates between two houses.

[In the case of] a wall between two houses and there is uncleanness within it, The house nearer to the uncleanness is unclean, And the house nearer to the clean part is clean. If [the uncleanness] is in the middle, both are unclean. In this section the wall serves to divide two houses, and not one house from the outside (as was the case in yesterday's mishnah). The rule is quite simple whatever house is closest to the source of impurity is impure and the other house is clean. If the impurity is right smack in the middle, the rule is strict and both houses are impure.

טומאה באחד מהן – one of the houses.

If there is uncleanness in one of the [houses] and there are vessels in [the thickness of] the wall: Those in the half nearer the uncleanness are unclean, Those in the half nearer the clean [house] are clean, And those in the middle are unclean. In this case, the source of impurity is in one of the houses and there are vessels in the wall. Again, the rule is simple. If the vessels are in the half of the wall nearer to the house with the uncleanness in it, the vessels are impure. If they are in the other side, they are clean. And if they are in the middle, they are clean.

מעזיבה (a concrete of stone chippings/clay covering the ceiling of the lower story and serving as flooring of the upper story) - a ceiling that separates between the house and the upper chamber.

[With regard to the] plaster-work between the house and the upper story, and there is uncleanness in it: If it is in the lower half, the house [below] is unclean and the upper story is clean If it is in the upper half, the upper story is unclean and the house is clean: If it is in the middle, both are unclean. Between the upper story and the bottom floor is a plaster ceiling (placed over wooden beams) and there is a source of corpse impurity in it. If it is in the lower half of the ceiling, then it is considered to be in bottom floor and the contents of the upper story are clean. If it is in the upper half, the upper story is unclean but the house is clean. And again, if it is in the middle, the rule is strict.

רבי יהודה אומר וכו' – But the Halakha is not according to Rabbi Yehuda.

If there is uncleanness in either [the house or the upper story] and there are vessels inside the plaster-work, Those in the half nearer the uncleanness are unclean, And those in the half nearer the clean [space] are clean. If they are in the middle, they are unclean. Again, the mishnah reverses the situation and this time the uncleanness is in either the upper story or house and the vessels are in the ceiling. The rule is the same as it has been in the other sections.

Rabbi Judah says: all the plaster-work is considered as part of the upper story. Just as Rabbi Judah said that the wall is considered to belong to the house (in yesterday's mishnah), here he says that the plaster-work is considered part of the upper story. Even if the uncleanness is in the lower half of the plaster-work, it defiles the upper story and not the house.