Answer
Article 5 TEU — conferral (the EU acts only within powers conferred), subsidiarity, and proportionality.
💡Subsidiarity: the EU acts only if objectives are better achieved at Union level (in non-exclusive areas).
Consolidated TEU (EUR-Lex) — eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:12016M/TXT