In this layer, messages are verified and interpreted, and data are stored in the Clinical Repository or retrieved. Applications are insulated from the implementation details of the Repository by means of this layer of data access modules (DAMs), that carry out the HL7 transactions. This architecture allows the physical aspects of data to be hidden from applications (e.g., whether the data is stored on a single machine centrally or distributed), and allows different database management systems to be used.
For example, one DAM is used to store the laboratory data in the Repository, one is used by an MLM to query
other data from the patient's records, and a third is used to retrieve some data requested by the outpatient system.