A Compound Unit is a relation between two Simple Units. Hence, before you create a Compound Unit, ensure that you have already created two Simple Units. For example, there are two Simple Units Nos (Numbers) and Doz (Dozen). Now define the relation between Nos and Doz, as one dozen equals twelve numbers.
Note: Use the Up arrow or Backspace key to highlight the Type field. Select Compound from the popup menu.
Select Compound from the Types of Units in the Type field. Select Doz in the First Unit from the Units list. Enter 12 in the Conversion field and select Nos in the Second Unit field. Accept Yes to save.
More:
Displaying and Altering Units of Measure