Using sourcing rules on drop shipment process

Summary:

We have implemented drop shipment process and some of the items are procured from multiple supplier sites. In that case we need to define these supplier sites on the sourcing rules separately, otherwise we have an error on the sales order.

The question is, what is the defaulting rule of sourcing rules on the order fulfillment lines? We have searched for a documentation but could not able to find any.

Should we define a supplier site as rank 1 on the sourcing rule to mark this supplier site as preferred one?

What is the usage of allocation percent on the sourcing rule for drop shipment sales orders?