Keilim, Chapter 24, Mishnah 11. Throughout this chapter the Mishnah takes one type of utensil after another and divides it into three, teaching that a single name can cover three different objects with three different levels of tum'ah. Here the subject is two familiar leather items: the chamat, a leather bottle or skin used for carrying liquids, and the tormil, the pouch in which a shepherd would keep his belongings while out with his flock.
Both of these were discussed earlier in the masechet, where we learned that their status depends on their capacity. A leather bottle or pouch that holds a certain volume is fit to be sat upon, and so it enters the world of tum'at midras, the tum'ah transmitted by a zav, a niddah or anyone in a similar state who presses down on an object meant for sitting or lying. One that holds less than that volume is too small to serve as a seat, but it is still a container, and containers have their own susceptibility.
The Mishnah begins by counting: "Shalosh chamatot veshalosh tormilin hein." Leather bottles come in three varieties, and shepherds' pouches likewise divide into three.
"Hamekabelin kashiur, temei'in midras." Whichever of them is roomy enough to hold the stated volume falls under the law of midras. For the leather bottle the volume in question is seven kav, and for the shepherd's pouch it is five kav. An item of that size is solid and wide enough that a person will lean his full weight on it and make use of it as a seat, and whatever serves for sitting is subject to the tum'ah pressed into it by a zav or a niddah.
The Mishnah continues, "Veshe'einan mekabelin kashiur," and rules of such items that they are "temei'in tumei meit." Where the capacity falls short of the measure, midras is ruled out, since no one would settle himself on so small a pouch. But the object has hardly stopped being a utensil. It holds its contents like any receptacle, and every receptacle is open to the tum'ah of a corpse and to the other forms of tum'ah that reach vessels.
"Veshel or hadag, tehorin mikolum." A bottle or a pouch sewn from fish hide remains tahor completely. No measure and no manner of use will render it susceptible, for whatever the sea produces never comes under the laws of tum'ah at all.
So the same pair of names yields three very different rulings: the large leather bottle or pouch that can serve as a seat and takes on midras, the small one that is only a vessel and takes on corpse tum'ah, and the one made of fish skin that stands outside the laws of tum'ah altogether.