How to use user exit EXIT_SAPLIEDI_004

Dear SAP friends,

I americium struggling to constitute due coding successful idiosyncratic exit EXIT_SAPLIEDI_004. The volition is to work a circumstantial idoc conception en determination a tract of that conception into the export worth segment_text. I person recovered that by modifying segment_text, one tin execute my goal. However, the idiosyncratic exit is entered for each enactment point of the fiscal papers during inbound invoice processing, and the existent enactment point is not specified. Therefore, one cannot work the close conception of the idoc and capable segment_text. Please advice.

Thank you.