In B2B the source will be internal but the item would be procured after the order is created or after the demand is made.
In Drop Ship the source will be external
In Drop Ship orders, material is directly shipped to the customer from the supplier. Thus, inventory is not affected. In this case, only logical receiving is done. But in the case of Back-to-Back orders, material is taken from inventory.
Drop Ship orders may have many Purchase Orders connected to them. In Back-to-Back orders one PO is tied to one Sales Order.