Port is the medium in which data is transferred.
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Download a nice presentation on IDOC, EDI , ALE
Configuration of EDI for the IDOC in case of standalone program
EDI Configuration : Define Logical Systems T.code SALE
Assign Client to the logical system T.code: SALE
EDI Configuration : Create RFC Destination Tcode: SM59
EDI Configuration : Define Port in transaction WE21
EDI Configuration : Maintain Partner Profile ( WE20 )