Light vs deep
The value split in color analysis: does your natural coloring sit at the pale end or the dark end.
Light versus deep describes where a person's natural coloring falls on the value axis, from very pale, light, to very dark, deep, independent of whether it leans warm or cool. In the twelve-season system this split defines four seasons outright: Light Spring and Light Summer sit at the pale end, both dominant-light despite one being warm and the other cool, while Deep Autumn and Deep Winter sit at the dark end, both dominant-deep despite the same warm-cool split. Value is closely tied to personal contrast, since a light-value person usually, though not always, has fairly low contrast between hair, skin and eyes, while a deep-value person often has more visible separation. Getting the light-versus-deep call right matters most for choosing how pale or how saturated-dark a season's neutrals and darkest shades should go, from cream and camel at one extreme to charcoal and true black at the other.
Why it matters
Light-versus-deep decides how far a person can safely go toward very pale pastels or very dark, near-black shades. A mistake on this axis makes even a correctly warm or cool color look drained on one end or overpowering on the other.