From the help screen:
Whse
Enter a warehouse code to use for this order, or click the Lookup button to list all warehouse codes. This field is available only if the Inventory Management module is integrated with Sales Order, and the Require Multiple Warehouses check box is selected in Inventory Management Options.
If there is a default warehouse defined for the customer ship-to address entered in the Ship To Address field, this field displays the customer ship-to warehouse.
If there is no default warehouse defined for the ship-to address entered, this field displays the default warehouse entered in the Warehouse field in the Default Values for Sales Order Entry window.
If a value is not saved in the Default Values for Sales Order Entry window, this field displays the warehouse entered in the Default Warehouse for Order/Invoice Header field in Sales Order Options. For more information, see Sales Order/Invoice FAQs.
If the Picking Sheet Only for Default Warehouse check box is selected in Sales Order Options, and the warehouse selected on the Header tab or for any of the inventory line items does not match the default warehouse defined in Sales Order Options, a message appears and the picking sheet is not printed.