Some IDoc types are supplied by SAP in the standard system: These are the basic types Other IDoc types are customer extensions: In these cases, a basic type is combined with an extension which is created by the customer, according to certain rules. Unlike customer extensions (or modifications, see:Customer Extensions, SAP and Customer Developments ), these extended basic types are upward compatible.
Different terms were used in 3.X Releases: The current basic type was previously called a "basic IDoc type" and an extension was called an "extension type". The term "IDoc type" was used to denote both the extended basic type and the umbrella term.
Extensions are "appended" to basic types via reference segments. Since basic types cannot lose segments in subsequent SAP releases, compatibility is ensured.