Look at the system unit in the units table. However ultimately it is up to you and how you configure the system:
System unit
Unit of measure that is applied when a measurement or quantity is entered in fields in Microsoft Dynamics AX that are not associated with a specific unit of measure.
Say that you have a company where kg is set as the system unit. In this company, any weight measurement in the Mass unit class, such as net weight, that is entered for an inventory item is identified as kg