About the 9th of the 15 FAIR principles, I1: (Meta)data use a formal, accessible, shared, and broadly applicable language for knowledge representation. You need controlled term sets, vocabularies, ontologies, thesauri, whatever you want to call it, ideally having globally unique, persistent, resolvable identifiers. And apart from these controlled vocabularies you need actual models, well-defined frameworks, to describe and structure the metadata according to those controlled vocabularies.