Keilim, Chapter 24, Mishnah 3. This chapter presents a series of utensils in sets of three, sorting each one into three possible statuses: a keli that can receive tumat midras, the tumah of something a zav or similar person sits or leans upon; a keli that can receive only tumat met, ordinary utensil tumah; and a keli that is tahor from every kind of tumah. Our mishnah applies this three-way division to the areivah, the kneading bowl in which dough is worked.
The mishnah opens: "Shalosh areivot hen" - there are three kinds of kneading bowls, meaning three halachic classifications of this one household vessel.
The cracked bowl: tamei midras
"Areivah mishnei log ve'ad tishah kabin shenisdekah, teme'ah midras" - a kneading bowl whose volume ranges from two log up to nine kav, and which has since split open, is susceptible to tumat midras. A crack ends its usefulness for dough, since it can no longer contain what is being worked inside it. A vessel of that bulk, however, is hardly discarded: flip it over and it makes a perfectly good stool. Sitting has now become what people actually do with it, so it joins the class of objects meant to be sat upon, and midras tumah attaches to it.
The whole bowl: tamei only with tumat met
"Sheleimah, teme'ah tumat met" - so long as the kneading bowl is sound and unsplit, it is subject to tumat met. It remains a usable implement, and every implement in service is open to ordinary tumah. Midras, though, does not reach it, since nobody manufactures a kneading bowl to sit on. Kneading is its purpose, and should someone perch on it while the dough was waiting, the household would send him off it, because the vessel cannot function with a person resting there. When sitting blocks a utensil from doing what it exists to do, that utensil is not classified as furniture for sitting, and the halachot of midras pass it by.
The oversized bowl: tahor entirely
"Vehaba'ah bemidah, tehorah miklum" - a kneading bowl larger still than the one described at the start of the mishnah is tahor from all tumah, whether it is whole or cracked. This follows the principle already established regarding oversized utensils: a keli of such dimensions is not a movable object, and only movable items can become tamei. Because it is not carried about but stays in place, no form of tumah takes hold of it at all.
So the single word areivah covers three very different halachic realities: intact and normal in size, it takes tumat met but not midras; cracked and no longer usable for dough, it becomes a seat and takes midras; and built on an oversized scale, it stands outside the world of tumah completely.