PDS lot size have confict with manually created planned order in SNP planning book

PDS lot size have conflict with manually created planned orders in SNP planning book:

1 The SNP PDS have fixed lot size transferred from PV.

2 The product have fixed lot size setting in the product view, the lot size value is the same with the PDS setting.

3 SNP heuristic planning do create SNP planned order with valid fixed lot size PDS.

4 Manually created planned order with multiple value of fixed lot size value do not have PDS selection screen popup, the planned order have no resource to select and remain blank, you have to maintain the related PDS in the detail view.

Here is the question:

what is the difference of the manually created and SNP heuristic planning logic?