ויקרא, פרק ט״ו, פסוק כ׳

The laws of ritual impurity establish clear boundaries regarding how a menstruating woman affects her physical environment, particularly the objects she rests upon. The primary approach among commentators is that the regulations governing a menstruating woman, as well as a woman experiencing an irregular discharge, are entirely identical to the rules for a man with a discharge. This complete parallel applies to how impurity is transferred to bedding, seating, riding saddles, objects that are moved, and bodily fluids [רש ר הירש, פירושי רד צ הופמן].

When examining how this impurity actually transfers to furniture, direct physical contact is not required. The deciding factor is simply the bearing of the woman's body weight. An object becomes impure the moment it supports her weight, regardless of how that weight is applied or whether there are intervening layers between her and the item [ביאור שטיינזלץ, רלב״ג].

However, this transfer of impurity does not happen with just any random object. Drawing a direct connection to the laws of a man with a discharge, commentators note that impurity only takes hold on items that are specifically designed and suitable for lying down or sitting. An object whose original, designated purpose is not meant for resting or seating remains entirely unaffected, even if she happens to lean or sit on it [אדרת אליהו, רלב״ג].