This section of IEC 870-5 applies to telecontrol equipment and systems with coded bit serial data transmission for monitoring and controlling geographically widespread processes. This section specifies rules for structuring application data units in transmission frames of telecontrol systems. These rules are presented as generic standards that may be used to support a great variety of present and future telecontrol applications. The layout is designed to limit the organizational overhead for standard data aquisition and supervisory control tasks to a necessary minimum with possible extensions for special tasks. From this point of view, it is appropriate to admit application specific or system specific choices of data presentation, of address structures and of chaining mechanisms for information objects in a frame. The corresponding arrangements can be, in most cases, assumed to be known by the communicating stations and thus need not burden the transmission frame. This section describes the general structure of application data without specifying details about information fields and their contents. It describes basic rules to specify application data units. Definitions and coding specifications of individual information elements that are frequently used in telecontrol applications are defined in IEC 870-5-4. Compatibility between devices of different suppliers can only be reached by defining complete application profiles. A complete application profile consists of: - the specification of the physical interface; - a subset of IEC 870-5-1; - a subset of IEC 870-5-2; - the specification of the application data units, based on both, IEC 870-5-3 and IEC 870-5-4; - the specification of the application functions based on IEC 870-5-5.