Comment 2 isn't stated very clearly.
Each option that can be selected for units must have a DvCodedString bubble pasted onto the value element.
When the CCD (XML Schema) is created. These will then be created as a group/choice.
Example (w/o expanding all of the DvCodedString elements):
Then in the DvQuantity units place it will reference the group: <xs:group ref="unitsgroup">
This forces the enduser to select one and only one option from the group.