Kelim, Chapter Twenty-Three, Mishnah Three
1) What is the practical difference between [the uncleanness of an object used for] riding upon and [one used for] sitting upon?
a) In the case of the former the effect of contact with it is different from the effect of carrying it, but in the case of the latter there is no difference between the effect of coming in contact with it or carrying it.
2) The pack-frame of a donkey on which a zav sat is clean;
a) But if the size of the spaces has been changed or if they have been broken one into another it is susceptible to uncleanness.
Section one: If one carries something that has contracted uncleanness by a zav riding upon it his clothes are unclean. But if someone touches something that has contracted uncleanness by a zav riding upon it, his clothes remain clean. Thus, when it comes to the uncleanness of something that was used for riding, there is a difference between carrying it and touching it. However, when it comes to something that has contracted sitting impurity (moshav) there is no difference between touching and carrying. In both cases, his clothes are unclean.
Section two: One does not normally sit upon the pack-frame of a donkey. Therefore, it is not susceptible to sitting impurity and it remains clean even if a zav sits upon it. However, if he changes the spaces or breaks them one into another such that he can sit upon it, then he has changed it into a seat and it is susceptible to sitting impurity.