CLASET is an EDI message designed to facilitate the exchange of tree structures such as classifications, code lists, organisational charts, concept definitions, catalogues, etc. The message has been designed in a generic way and provides mechanisms to describe the nature of the information exchanged within it. All the implementation and software tools are based on a unique Conceptual Data Model (CDM). This CDM can be used to define all information needed about the message and three main data structures: classifications, tables of links between two classifications and updates. This CDM can be used not only as an exchange message, but also as a database model. CLASET is presently implemented in four formats: XML, SGML, UN/EDIFACT and HTML; this list is not at all finite: new syntaxes might come under consideration for inclusion in the message, with the move to yet unknown technologies.