Use when: extracting, reusing, or naming hooks, state management, or other component-agnostic behavior in Primer React, or when a component's behavior hook wraps a native element with built-in behavior such as `<dialog>`. Covers the compound behavior-hook pattern (prop-getters), the controlled component contract for wrapping native elements, the difference between a component's own hook and a generic reusable utility, naming conventions, and where utilities live in the package.