Kelim, Chapter Twenty-Four, Mishnah Four
There are three different types of boxes:
1) A box whose opening is at the sides is susceptible to midras uncleanness;
2) If it is on the top it is susceptible to corpse uncleanness;
3) And if it holds the prescribed measure it is free from all uncleanness.
Section one: This was explained above in 19:9. Since this box can be sat upon, it is susceptible to midras impurity.
Section two: See 19:7. Since this box cannot be sat upon, it cannot contract midras impurity. But it is a vessel so it is susceptible to corpse uncleanness.
Section three: If it holds more than 40 seahs, it is not susceptible to impurity at all (see the end of yesterday’s mishnah).