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 usually use um and PCB processes usually use mm or mil. Previous versions allowed multiple technologies to be 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.
See 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 wire bonded out to a board to complete the output matching network.
The top level schematic in this design combines the two pieces together. Notice the units of this schematic for length are mils as you can see by the bond wires.
If you push into the 1Stage_Amp subcircuit, you will see the length is now um. The view below is only some of the components in that schematic.