When you scale a complex Kanji character with 20+ strokes up to a W9 weight, the strokes threaten to bleed into each other, turning the character into an illegible black blob. Hiragino Sans W9 solves this through several design triumphs: 1. Optical Adjustment and Stroke Thinning

In short: is the boldest, heaviest weight of the standard Hiragino Sans typeface.

Regular to Medium-Bold (the workhorses of body text and subheadings). W7 to W8: Heavy (used for emphasis and titles).