Mark Greenwell wrote:
...Does anyone know the reason why in the document units (custom units) there isn't an option to use Ton's or Tonnes for the weight.
To answer your question,because there has been little or no user request/demands for that unit.
Additionally, there is a common workaround in adding the conversion factor to a custom property or design table to get the desired output.
Look at it a different way, should SW have the option for every conceivable unit ever used? That would make for a long list.