Data exchange system includes at least one portable data processing unit with data communication processing and memory units, the latter including an executive program and one or more application descriptors, each application description including at least one interaction context comprising commands, data elements, data references, procedures, access conditions, and external references; the structure of the data elements and the data references as well as other references is chosen in such a way that a very efficient use of the restricted memory space of e.g. smart cards is obtained.