Field help (PUU) 

This is the purchase unit defined for the product-supplier combination, or for products that do not exist in the product-supplier record.
This product has two purchase units:

This purchase unit cannot be modified. It is used as the default value when entering an order, subcontract order or receipt. The supplier purchase unit is suggested by default when the product-supplier record exists.

The purchase units are accessible depending on the type of category assigned.

SEEINFO Upon invoicing a product, the product purchase unit is used.