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.