Depending on file type, the part might not have a defined unit. For those parts the application tries to determine the unit depending on the relative size to the Print Platform of the current printer.
Display unit
Choose display unit in the Preferences. It sets which unit is displayed and used throughout the application.
Unit alert message
If the unit of a part cannot be determined, a warning message will appear.
Check the size comparison and choose which unit is correct.
Click Set.
Convert part unit
Use the Scaling by Unit command 
to convert the part to another unit.
