Use when: deciding how to structure a new Primer React component, evaluating whether an existing component's API is at the right level of abstraction, or orienting on the modular design system approach before using a more specific modular-ds-* skill. Covers how the four API types (config, presentational, base, utilities) relate and compose, and how to decide which ones a component needs. The model itself is defined in contributor-docs/style.md.