In the current specification, the EXTRACT_CHAPTER class has a mandatory entity_identifier attribute, which identifies the entity (usually patient or other demographic entity) whose data are contained in the chapter. However, a more flexible use of the model would allow some chapters to be used for purposes like common demographics.
The modelling of entity_identifier is also problematic. Within the EXTRACT_ENTITY_MANIFEST class, it is hard-modelled to consist of openEHR-style Ehr id and subject ids, but it would be more practical to use String identifiers, and also to allow more kinds of identifier.
The method of reference from EXTRACT_CHAPTER to EXTRACT_ENTITY_IDENTIFIER is also impractical, particularly in serialised expressions of an extract.