supplemental

This term suggests a relationship of dependency where the addition is not the primary focus but serves to fill a gap or strengthen a foundation. It carries a neutral to positive connotation of thoroughness and completeness, often appearing in formal, legal, or academic settings to indicate that the original source was insufficient on its own. While complementary implies two things that work well together to create a whole, supplemental specifically implies an add-on. It is the linguistic equivalent of a patch or an extension, used when the main entity exists independently but requires extra support to reach full utility or coverage.