Item grouping of F.19/ GRIR clearing

Hello,

I'm just trying to figure out after what criterias the T-Code F.19 is grouping the open items together but don't find an answer. SAP help is saying "for each reconciliation account and assignment number..." but I don't find an assignment number. I tried the PO, the supplier id, a mixture of both but I'm not able to assign the items of F.19 to the posting items of F.19.

Can somebody please give me a hint?

Thanks a lot in advance.

BR,

Hansi