This page of the Preferences dialogue box is used for miscellaneous preferences that don't merit a page of their own. The page is accessed by selecting the Tools | Preferences menu item then clicking on Misc. in the list on the left of the window.
There is only a single option that can be set on this page.
Select the measurement units to be used by the application from the Preferred units of measurement drop down list. Options are Inches and Millimeters. If you have not set this option before, the measurement units will be appropriate to your locale.