Keilim 23:2-3

Keilim 23:2

The following things are susceptible to impurity as objects upon which one rides: an Ashkelon donkey belt, a Medean saddle, a cushioned camel saddle and a horse blanket. Rabbi Yosi says that a horse blanket is also susceptible to impurity as a seat because people stand on it in the arena. But a female camel’s saddle is susceptible to impurity. [It’s unclear whether this last clause is part of Rabbi Yosi’s statement or a response to it, and therefore whether it refers to riding or sitting impurity.]

Keilim 23:3

The difference between things upon which one rides (merkav) and things upon which one sits (moshav) is as follows: touching a merkav is different from carrying it but touching a moshav is the same as carrying it. The pack frame of a donkey upon which one sits is insusceptible to impurity (because it’s not really fit to sit on) but if one changed the spacing between the wooden slats or removed one to make more room, then it is susceptible.