From: Rabbi Moshe Sofer
To: Rabbi Lipman
The Chasam Sofer distinguishes three categories: a passive stain on a garment requires the larger Talmudic k'gris v'od measure; a stain found via an unchecked cloth with a sensation of liquid flow requires the stricter modern louse measure; and a genuine bodily hargashah such as uterine opening renders a woman impure with any amount at all, removing her chezkas taharah entirely. He disputes the Noda BiYehuda's view that zibbas davar lach constitutes a full Torah-level hargashah.
Source: Shu"t Chasam Sofer YD §150