What is composition aggregation?

Example: You're building a project brief. The executive summary is contained — it lives inside the document, dies if the document is deleted. The link to the market research report is a reference — it points elsewhere, and the research exists independently. If you embed the entire report inside the brief, you get a 40-page monster nobody reads. If you only link to the summary without containing the key conclusions, reviewers miss the point. The discipline is knowing which pieces belong inside the boundary and which belong behind a pointer.

This concept is part of Hierarchy and Nesting in the How to Think curriculum.