Application Link Enabling (ALE) is a mechanism for the exchange of business data between loosely-coupled R/3 applications built by customers of SAP, the enterprise resource management program. ALE provides SAP customers with a programme distribution model and technology that enables them to interconnect programmes across various platforms and systems. There are three layers in the ALE system: application services, distribution services, and communication services. The vehicle for data transfer is called an IDoc (intermediate document), which is a container for the application data to be transmitted. After a user performs an SAP transaction, one or more IDocs are generated in the sending database and passed to the ALE communication layer. The communication layer performs a Remote Function Call (RFC), using the port definition and RFC destination specified by the customer model. The IDoc is transmitted to the receiver, which may be an R/3, R/2, or some external system. If the data is distributed from a master system, the same transaction performed by the sender will be performed by the receiving system, using the information contained in the IDoc. Changes made to fields in master data tables can be set to trigger distribution of the changes to slave systems, so that multiple database servers can update the same information simultaneously. IDocs carry information directly between SAP systems. In order to communicate with a non-SAP system, an IDoc is first transmitted to an intermediary system that translates the data to a format that will be understood by the receiver. Return data also passes through the translating system, where it is again encapsulated into an IDoc.