Metadata registries comprising sets of categories to be used in data collections exist in many fields. The purpose of a metadata registry is to facilitate data exchange and interoperability within a domain, and registries often contain definitions and examples. In this paper we will argue that in order to ensure completeness, consistency, user-friendliness and extensibility, metadata registries should be structured as taxonomies. Furthermore we will illustrate the usefulness of using terminological ontologies as the basis for developing metadata taxonomies. In this connection we will discuss the principles of developing ontologies and the differences between taxonomies and ontologies. The paper includes examples of initiatives for developing metadata standards within the field of language resources, more specifically lexical data categories, elaborated at international and national level. However, the principles that we introduce for the development of data category registries are relevant not only for metadata registries for lexical resources, but for all kinds of metadata registries.

We are working on a project called CAOS - Computer-Aided Ontology Structuring - whose aim is to develop a computer system designed to enable semi-automatic construction of concept systems, or ontologies. The system is intended to be interactive and presupposes an end-user with a terminological background (terminologist or professional translator). CAOS supports terminological concept modelling. The backbone of this concept modelling is constituted by characteristics modelled by formal feature specifications, i.e. attribute-value pairs. Our use of feature specifications is subject to a number of principles and constraints. In this paper we want to demonstrate some of these principles and to show why they are necessary in order to permit the construction of an interactive tool for building terminological ontologies. We will also show how they contribute to determine the structuring of the ontologies in CAOS and to facilitate the work of the terminologist user.