Field help (VALMTHD) 

If the Product field is blank, enter or select a valuation method code to include all products in the selected Site assigned to this valuation method.

If you enter a Product, this field defaults to the valuation method assigned to the product. You still have the option to enter a different valuation method if, for example, the selected product changed valuation methods during the period and you want to see values for the prior valuation method.