ויקרא, פרק ט״ו, פסוק כ״ב

When a menstruating woman sits on an object, the state of ritual impurity transfers to that item. Anyone who touches it also becomes impure. To become pure again, the affected person must wash their clothes, bathe in water, and wait until evening.

There is a slight difference in how this rule is presented compared to the similar laws for a man with an abnormal discharge. For the man, the instructions explicitly mention what happens if another person sits on the affected object. In this case, however, the focus is only on someone who touches the item. The reason for leaving out the act of sitting is simply a matter of placement. The specific details regarding a person who sits on an object the woman previously sat on are not ignored, but rather fully explained in the very next set of instructions [העמק דבר].