how to match the segment fields of EDI

Hello experts,

please advise how I can match the segment fields of an IDOC with the right fields in the application, so that the fields will be filled with the right content.

Where is the right spot in the customizing to do that? For example E1EDK09-VTRNR of DELFOR is filled with the scheduling agreement number.

But where is the place that says that this field in this segment should be filled with this number?

Thanks in advance

Stefan