Commentaire sur Nida 6:3

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.”

Whatever is susceptible to midras uncleanness is also susceptible to corpse-uncleanness. For an item to be susceptible to midras uncleanness, which is transmitted by sitting, laying down or leaning upon something, the item must have been made to be sat upon or laid upon, for instance a mattress or a chair. Therefore, not all items are susceptible to midras uncleanness (for instance, my shirt is not susceptible). However, all vessels (this includes almost anything of practical use made by a human being) are susceptible to corpse uncleanness. So if an item, such as a sofa, is susceptible to midras uncleanness, it must also be susceptible to corpse uncleanness.

But there are things that are susceptible to corpse uncleanness but not to midras-uncleanness. Such as my shirt, or my cereal bowl (or your dress and serving platter, if you like).