Commentary for Kelim 16:8
תִּיק הַסַּיִף וְהַסַּכִּין וְהַפִּגְיוֹן, תִּיק מִסְפֶּרֶת וּמִסְפָּרַיִם וְהַתַּעַר, תִּיק מִכְחוֹל, וּבֵית הַכּוֹחַל, תִּיק מַכְתֵּב, וּתְרוּנְתֵּק, תִּיק טַבְלָא וּסְקֹרְטְיָא, בֵּית הַחִצִּים, בֵּית הַפָּגוֹשׁוֹת, הֲרֵי אֵלּוּ טְמֵאִים. תִּיק סִמְפּוֹנְיָא, בִּזְמַן שֶׁהוּא נוֹתֵן מִלְמַעְלָה, טָמֵא. מִצִּדּוֹ, טָהוֹר. תִּיק חֲלִילִין, רַבִּי יְהוּדָה מְטַהֵר, מִפְּנֵי שֶׁהוּא נוֹתְנוֹ מִצִּדּוֹ. חִפּוּי הָאַלָּה, הַקֶּשֶׁת, וְהָרֹמַח, הֲרֵי אֵלּוּ טְהוֹרִין. זֶה הַכְּלָל, הֶעָשׂוּי לְתִיק, טָמֵא. לְחִפּוּי, טָהוֹר:
The sheath of a sword, a knife or a dagger, the case for scissors, or shears or a razor, the case for 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 impurity. The case for a double flute is susceptible to impurity if the instrument is put in from above, but if it is put in from the side it is pure. A case for flutes—Rabbi Yehudah declares it is pure because he puts it in from the side. The covering of a club, a bow or a spear are all pure. This is the general rule: that which serves as a case is susceptible to impurity, but that which is a covering is pure.
Bartenura on Mishnah Kelim
English Explanation of Mishnah Kelim
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.
A case for flutes Rabbi Judah says it is clean because these are put in from the side.
The covering of a club, a bow or a spear is clean.
This is the general rule: that which serves as a case is susceptible to uncleanness, but that which is merely a covering is clean.
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 example a 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.