The field lineitem_export_id corresponds to the field POS_ZEILE of the DSFinV-K legal documentation and is described as follows:
POS_ZEILE
Position number within a slip (must be unique and consecutive).
Good examples and explanations of the different fields and business cases can be found in the DSFinV-K documents of the Federal Central Tax Office (Bundeszentralamt für Steuern):
DSFinV-K 2.3, (new version) (EN)
DSFinV-K 2.3, Stand März 2022 (DE).