This kit uses one family, Cormorant, for everything but code. It has more range than that sounds, because a good book face holds several voices.
Light is for size
Cormorant was drawn as a display face, with fine hairlines that come alive at large sizes. Headings use its light weight and let scale do the work. Set a title at 60px in light and it needs no bold to be the loudest thing on the page.
Medium is for reading
The same hairlines thin out at small sizes, so body text moves up to medium and turns on the Garamond letterforms, whose a and e have larger counters. It also sits two sizes above where a grotesque would, because the x-height is small.
Italic is for asides
Leads, captions, quotations and the current page are italic. That gives a page a second speaker without a second font.
A good book face holds several voices.
Semibold is for names
A list item’s name and every small label are semibold. Those are the places a reader scans, so they get the firmest strokes the family has.