Keilim, Chapter 23, Mishnah 2. This mishnah continues the discussion of items that a zav renders tamei by resting his weight on them, and it draws a line between two different categories of that tum'ah: the tum'ah of a seat and the tum'ah of a riding-place.
A word of background is needed. Tum'at midras attaches to an object only when that object was manufactured with the intention that a person rest his body on it. If a zav sits down on a utensil whose designated function is to support someone sitting or lying, the utensil takes on midras tum'ah, the same tum'ah that the Mishnah elsewhere labels with the terms mishkav (bedding) and moshav (seating). But if the zav sits on an item never meant to hold a resting body, midras does not attach to it at all.
There is a second branch of this tum'ah, called merkav, an item made for riding. When a zav places his weight on something designed for riding rather than for sitting, the object becomes tamei as a merkav and not as a moshav. The practical differences between a merkav and a moshav are spelled out in the mishnah that follows; here the Mishnah simply lists which items fall into the category of merkav.
The list
The Mishnah opens: "eilu tamei'in mishum merkav," these are the items that become tamei as a merkav and not as a moshav.
"Zeriz ha'Ashkeloni": a saddle strap of the type made in Ashkelon, produced from wool and fastened onto the animal's back. Since its purpose is riding and not sitting, a zav who leans his weight upon it renders it a merkav rather than a moshav.
"Umedochah ha'Madit": a mortar of the kind made in Madai. This was a large wooden block hollowed out into a bowl at the top, and a person would pound with a pestle into that bowl to crush the spices placed inside. Because it is a solid block, a person would seat himself upon it while he pounded, sitting astride it in a manner unlike ordinary riding. For that reason it enters the category of merkav and not of moshav.
The list continues with "akef shel gamal", the saddle placed on a camel, and "tafitan shel sus", the cloth saddle spread on a horse. Neither one was produced so that someone could sit on it; both exist so that a person can ride. Accordingly, a zav who presses his weight down on them makes them tamei as a merkav, not as a moshav.
Rabbi Yose's objection
The Mishnah now records a dissenting opinion. Rabbi Yose maintains that "af tafitan shel sus", even the horse's saddle-cloth, is "tamei mishum moshav", capable of receiving the tum'ah of a seat as well. His reason: "mipnei she'omdin alav bakumpon", at the royal games people would climb up and stand on it. Standing on the cloth gave a spectator the extra height he needed to follow the contests from a distance, and once an object serves as a place upon which a person stands, and not only as gear for mounting an animal, the tum'ah of a moshav can take hold of it too.
"Aval ukaf shel na'akah tamei": here Rabbi Yose concedes. The saddle of a she-camel remains tamei only as a merkav and not as a moshav, since it is used exclusively for riding and never as a place upon which a person stands.
The Mishnah thus teaches us that the classification of an object under midras tum'ah follows the use for which people actually employ it: a strap, a mortar-block or a saddle used for mounting and riding falls under merkav, while an item that also serves as a platform to stand upon can be drawn into the category of moshav.