Komentarz do Kapłańska 15:9

וכל המרכב AND ANYTHING FOR RIDING — even though he has not sat upon it when riding, e. g. the hold in front of the saddle, which is called arcon in old French, becomes unclean as a part of something for riding on (משום מרכב), and the אוכף, saddle, which is called alves in old French, is unclean as being something to sit upon (משום מושב) (Eruvin 27a).

Even if he had not sat. Meaning: He does not actually sit on it, which would be sitting mentioned in the verse (6). Rather, [this refers to] the side of the saddle, which is the wood in the front and back of the saddle that he does not sit upon, but rather the rider holds onto it so he will not slip forwards or backwards. Therefore, it is called the ‘hold,’ since he holds onto it.

That is that He commanded us that a zav be impure. And this commandment includes the laws of the things that would make him a zav and the description of his transmitting his [impurity]. (See Parashat Metzora; Mishneh Torah, Those Who Defile Bed or Seat 6.)