A family loaded inside another family - a handle family nested inside a door family, a hinge nested inside a window - letting complex components be built out of smaller, independently maintained pieces rather than one monolithic geometry blob. The parent family can expose (or "shared") the nested family's parameters so they still appear and schedule correctly at the project level.

Nesting is what makes large, realistic families genuinely manageable: update the handle family once, and every door family that nests it picks up the change, rather than needing forty separate handles remodeled by hand.