You can control whether the BAPI should be called synchronously via RFC or asynchronously via ALE by the<referenceID> element in the BizTalk header.

If the <referenceID> element is specified and contains a valid SAP transaction ID (TID), the BAPI outbound process automatically chooses the ALE format, otherwise, if the element is omitted, the message will be processed synchronously.

For both synchronous and asynchronous calls, technical processing errors are reported by an XML response document containing a fault descriptor.