Since no Shipping Method has been assigned at this point, the Shipping Type defaults to Pickup. As a result, the new document's initial Tax Schedule ID is the Tax Schedule ID specified in the Site Maintenance window for the above mentioned Site ID.
When a Customer ID is assigned to the document by setting the DocCustomerID property, the customer's Ship To Address ID is automatically used.
If the Shipping Method specified in the Customer Address Maintenance window is not blank, then the document's Shipping Method is updated with this value.