Basic Type, Enhancement, IDoc Type
Some IDoc types are supplied by SAP in the standard system: These are referred to as basic types. Other IDoc types are customer enhancements: In these cases, a basic type is combined with a customer-defined enhancement according to certain rules. Unlike customer enhancements (or modifications, see Customer Enhancements, 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 enhancement was called an "enhancement type". The term "IDoc type" was used to denote both the extended basic type and the umbrella term.
Enhancements are "appended" to basic types via reference segments. Since basic types cannot lose segments in subsequent SAP releases, compatibility is ensured.