Kelim, Chapter Sixteen, Mishnah Eight
Our mishnah deals with cases for weapons and musical instruments. Sections one through five bring various examples of cases, but the explanation for the entire mishnah comes in section five. If the case is used for storage then it is susceptible. This means that it is something that the vessel is put into, such as the sheath of a sword. But if it is merely a “covering” and not a “container” it is not susceptible. To use a modern examplea bread box would be susceptible, but a hallah cover would not.
In section two, if the double flute is put in from the top, then the case is considered to be a container and it is susceptible. However, if it is put in from the side, then it is considered to only be a covering.
I think the rest of the mishnah is understandable in light of the general rule in section five.
1) The sheath of a sword, a knife or a dagger, the case for scissors, or shears or a razor, the case of make-up and the make-up box, the stylus case, the compass case, the tablet case, the case for a board, a quiver and a javelin case all these are susceptible to uncleanness.
2) The case for a double flute is susceptible to uncleanness if the instrument is put in from above, but if it is put in from the side it is clean.
3) A case for flutesRabbi Judah says it is clean because these are put in from the side.
4) The covering of a club, a bow or a spear is clean.
5) This is the general rule: that which serves as a case is susceptible to uncleanness, but that which is merely a covering is clean.