With StarTrack you are required to specify a Unit Type for each package. The Unit Type will be one of the following:
The Unit Type is specified in StarShipIT by using the Packaging or Boxes which is setup under Settings > Package Setup. For any box that you wish to use with StarTrack you must add a Unit Type, and this can be done by selection from the drop down box.
When you are shipping an order using the dialog then you can select from the list of packaging types or boxes, and the unit type of that box will be used for the package sent. If you batch print, it will use the unit type of the package pre assigned to the shipment.
Below is a screen shot of the Box setup as seen under Settings > Package Setup: