Field help (ITMCUSUOM) 

Enter an additional unit only if required by the product commodity code.

Additional units are units of measure different from the kilogram, such as shoe size, liter, gram, etc.
The units of measure must not contain any decimals nor an indication of the units. Quantities are rounded to the nearest unit, except for additional units between zero and one that are always rounded to one.

Such a behavior is for information purposes only and is not directly used in the application. The unit used is the EU unit defined on the Products record from the Units section.