If you look at, for example, the German VBAP table, POSNR is “Positionsnummer des Vertriebsbeleges”, so yes, POSNR is POSition NumbeR.
In SAP tables, it is very often used in combination with doc. Number for creating a primary key for items or subdocuments, especially in SD (sales) tables. Thus, you have VBAK-VBELN for the header of the purchase order and VBAP-VBELN + VBAP-POSNR for the items in the purchase orders. So, in LIKP / LIPS, VBRK / VBRP, VBUK / VBUP ...
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