Rashi on Leviticus 15:23/רש"י על ויקרא ט״ו:כ״ג

1ואם על המשכב הוא AND IF ONE BE ON HER PLACE OF REPOSE [OR ON ANY THING WHEREON SHE SITTETH] — This means, one who sleeps or sits on the place where she has slept or has sat even though he does not directly touch it (e. g. if there are covers upon the couch or the seat; for if he touches it vv. 21 and 22 apply); he also comes under the law of uncleanness that is stated in the above verse — that he requires washing of garments.

2על הכלי UPON A VESSEL — This is intended to add to משכב and מושב any article for riding on (Sifra, Metzora Parashat Zavim, Section 4 15).

3בנגעו בו WHEN HE TOUCHETH IT — "it" (בו) — this refers only to מרכב which, as has just been stated, is included in the law on account of the words על הכלי.

4בנגעו בו יטמא WHEN HE TOUCHETH IT HE SHALL BE UNCLEAN, but does not require washing of the garments, because so far as the מרכב is concerned its being touched does not communicate uncleanness to people so that they in turn should render their garments unclean (cf. Mishnah Kelim 23:3).