Unibody 8 contains five fonts: Regular, SmallCaps, Italic, Bold and Black.
Regular is wide and open to give good legibility in on-screen body text.
Italic is upright and more narrow than Regular, and it clearly stands out in a roman text.
Bold and Black are designed to emphasize single words within Roman-style. The black weight is a bit more detailed than the rest of the fonts, and works well as an 8 pt. 'display' font.
SmallCaps are intended to set capitals beside each other, for example in chapter headers, in titles, acronyms or simply for accentuating text within a roman body.