Starting in version 15, you can define units per schematic. This feature is helpful when putting together multi-technology designs, specifically for length units where IC processes are usually us um for the length and use um and PCB processes usually use mm or mil for the length unitsmil. Previous versions allowed multiple technologies to be use used in terms of layout but the units had to be one set for the entire design. For simplicity's sake, if you typically would use um and mm for your length units, you would have to pick one which would make the values for the other technology less intuitive.
This is best shown with an exampleSee the following example to help illustrate the concepts. This comes from the AWR_MMIC_Module shipping example.
In this design, there is a GaAs IC for the input match and transistor which is then bondwired wire bonded out to a board to complete the output matching network.