John Gill Commentary Leviticus 15:6

"And he that sitteth on anything whereon he that hath the issue sat shall wash his clothes, and bathe himself in water, and be unclean until the even." — Leviticus 15:6 (ASV)

And he who sits on [any] thing on which he sat that has the
issue
Shall be unclean, even though he does not touch it. Jarchi says, though there should be, as he adds, ten things or vessels one upon another, they all defile because of sitting, and so by lying:

shall wash his clothes, and bathe himself in water, and be unclean
until the evening; as in the preceding case. (See Gill on Leviticus 15:5).