The structure of the GALEN Common Reference Model consists of four parts:
- The high level ontology – sometimes called the high level schemata which describes the overall structure – the sorts of concepts and the broad patterns by which they can be composed to produce more detailed concepts.
- The Common Reference Model itself – the re-usable parts of anatomy, surgical deeds, diseases, clinical signs, etc., their definitions, descriptions, and the constraints on fitting them together. The Common Reference Model itself is broad and shallow and expected to be shared by most applications (although it is highly modular internally and can be subdivided if needed.)
- Detailed extensions to the building blocks required for specific applications or specific subdomains – more detailed information on specific body regions, types of surgery, etc, required in a particular subdomain of surgery, e.g. cardiovascular, respiratory, urologic, etc.
- The model of surgical procedures and other similar models which define composite concepts made up of the parts from the Common Reference Model and its extensions.
There is no sharp principled dividing line between these four levels. They are defined methodologically and organisationally rather than by a set of strict logical criteria. A model in the GALEN Representation and Integration Language has no sharp dividing line between levels of concepts, only between concepts in the model and the individual patients, their conditions, and the operations actually performed on them. There is a continuum between high level concepts such as Phenomenon or disease and highly specific concepts such as brittle insuline dependent diabetes mellitus or between body part and distal phalanx of the left fourth finger.