The basis for all communication between SAP and external components (as well as for most communication between SAP components) is the Remote Function Call (RFC) protocol. Most client programs want to use regular, Synchronous RFC (sRFC), but SAP also supports Transactional RFC (tRFC) and Queued RFC (qRFC). tRFC is used mainly to transfer ALE Intermediate Documents (IDocs).
ABAP Function Modules can only be called from an external client if they are marked as RFC-enabled. R/3 contains several thousands of such RFC-enabled Function Modules (RFMs). Amongst them are the BAPIs. BAPIs are RFMs that follow additional rules and are defined as object type methods in SAP's Business Object Repository (BOR).
Most BAPIs are released RFMs, only very new ones may be unreleased at first for testing before the releasing. On the other hand, there are quite a few extremely useful RFMs that are not officially released for customer use. (From sap.com)[@more@]
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