Partial Purchase Orders feature is available in QBO plus now.
You can create a purchase order for both accounts and items tabs. Item tab is more for inventory items. Account tab is an expense that adds directly to Profit and Loss. You can now receive partial shipments and the PO will remain open for back ordered items.
Create a Vendor Bill (or Check, or Expense) form. Select a Vendor, a drawer at the right panel will open. Select add to the bill. You can select to choose either partial or complete order. For back orders, it will show as back ordered quantity and open balance amount for inventory items on a PO. For accounts, it will show open balance amount. The Purchase Order will show the status as open. You can run Open Purchase Order Detail report and Open Purchase Order List report for detailed information.