That would be where the breasts themselves have two different colors that create a shading pattern. If the breasts as a whole are a single color, which is distinct from the rest of the torso, that's not countershade breasts (or necessarily countershading, depending on the overall pattern for countershade_torso).
For post #5550618 specifically, perhaps countershade_chest would be most applicable, since the white coloring also goes to the upper chest and neck, and a little bit underneath the breasts. The breasts contribute to the upper torso having a countershade pattern, but since the breasts themselves are wholly white, there's no countershading on the breasts themselves (this is part of why I think these more specific countershade_<bodypart> tags need to be aliased away; they're often misused to mean <bodypart> is a separate flat color, perhaps contributing to the countershading of what they're part of, but doesn't itself have a countershade pattern).
Updated