Hy,
so I just did investigate the purchase requisiton.
Purchase requisition of the type "consumption" does no interact with any inventory demand and so the purchase order will be cretaed automatically or manual for each line of a PR.
To optimize Purchase Order against your vendor "Consolidation opportunites" can bring only the same item from muldiple PR's to one purchase order with one line for every PR line.
This post shows what happens; organicax.com/.../
If you cretae the PR with the option "Replenishment" then the demand for the PR will go through the inventory logic and is calculated based on the settings for master planing. On this way the demand will create as many "planned purchase orders line" as PR's which you can firm into one purchase order with one line for each item, and the summarized quantity of all PR's of this item.
My PR:

My PlannedOrders;

My PO after master planing and firming:

Hope this helps.