Product Units
Product units are a list of the allowed units for the product. If the list is blank you can use any product unit. If there are product units listed, then only those units can be used with the product. Refer to for usage details Understanding product unit.
If you do not have any entries on this tab on the product. Then users can select any unit they like when selling the product. This can result in some large errors.
Normally users don't change the unit on sales or purchasing. So this is not an issue. But it can be very important for products where the users do change the sell unit.
If you have a product, particularly one you setup manually, where users may want to change the sell unit in particular. Make sure you list all the units you will allow them to use on the product units tab on the product.