You basically answered this yourself in the thread title. CAD systems use "units" in creating geometry. When the systems are set up, you configure these "units" to mean either inches or mm. When creating the toolpaths in CAM, a post processor will use these units to translate ( not 'convert',... thats another story ) to inches or mm output. So in if you have a system configured in metric, your coordinates 10,50 to 10,100 in X/Y will place these points at : X10 mm, Y50mm and X10mm, Y100mm. If configured for inches it would be X10.000, Y50.000 inches and X10.000, Y100.000 inches.
The units that are displayed when analyzing the geometry and/or cut will depend on what the system was configured for to begin with. Which is why in many systems, its important to know that when importing geometry, was it created in inches or metric? For example: say you have a part that drawn on a metric system and you have a box that is drawn at 1mm square. If you import this geometry into an "inch" system, it will show up in the inch system as a 1" square. This is obviously a considerable difference.
Now some systems will allow you to have it converted when importing. Others you will need to scale the geometry to your system once its imported.
It's just a part..... cutter still goes round and round....