BAPIs of SAP Interface Types 
BAPIs of SAP Interface Types
Use
As of Release 4.5A, BAPIs can also describe interfaces that are implemented externally (not in an SAP System), and that can be called in external systems by an SAP System. These BAPIs are known as BAPIs used for outbound processing. The target system is determined for the BAPI call in the distribution model of Application Link Enabling (ALE).
BAPIs used for outbound processing are defined in the Business Object Repository (BOR) as API methods of SAP interface types. In contrast to BAPIs of SAP business object types, the BAPIs in an SAP System are not implemented as function modules.
Functions that are implemented externally can be standardized and made available as BAPIs.
With the exception of the above mentioned points, the information in the BAPI User Guide and the
BAPI Programming Guide refers to the BAPIs of both SAP interface types and SAP business object types. If this is not the case, the documentation will explain what the differences are.