Field help (PTHUOMSTU) 

This is the conversion coefficient between the receipt unit and the stock unit. This information can be accessed for packing units if the setup authorizes it (refer to the packing unit grid on the Units section of the Products record). This information cannot be accessed for the other units (product purchase unit, supplier purchase unit, supplier packing unit).
If the product has the sub-contract type, the conversion coefficient between the receipt unit and the stock unit can never be modified.