Mishnayos Niddah Perek 6 Mishnah 3

כל המיטמא במדרס – the person with gonorrhea. All utensils that is worthy of becoming a primary source of ritual impurity through the Levitical uncleanness arising from a person with gonorrhea’s immediate contact by treading on/leaning against something, as for example a utensil that is designated for lying and/or sitting if a person came in contact with the dead or was defiled in the tent of a corpse. And there is a utensil that completely becomes susceptible to receive all defilements and becomes a primary source of ritual impurity through [contact with] the dead corpse but does not become a primary source of ritual impurity through the person with gonorrhea through lying and/or sitting, as for example, if the person with gonorrhea bent over/turned upside down a Seah and sat upon it, or a field requiring a Tarkav (i.e., a dry-measure, three Kabs) and sat upon it, that very utensil is not impure through treading to become a primary source of ritual impurity, but rather first-degree ritual impurity through its contact with a person with gonorrhea, as it is written (Leviticus 15:4): “[Whoever sits on an object] on which the one with a discharge has sat [shall wash his clothes, bathe in water], and remain impure [until evening],”he who is designated for sitting, excluding this one that they say to him: “Stand and let us our work.” But if he came in contact with a dead corpse, he becomes a primary source of ritual impurity, for we don’t say with someone who is defiled through contact with the dead, “Stand and let us do our work.”