In the general preferences it says I can choose to display measurements as either "Screen pixels" or "Window points." What's the difference?
 

Starting with version 3.5, xScope fully supports the Retina display. One of the great things about the Retina display is that you can no longer see pixels. Yet at the same time, pixel precision in user interfaces is as important as ever.
 

If you're a long time user of xScope, you may be surprised at how little the app has changed on the surface. There is now a General preference setting that lets you choose "Screen pixels" or "Window points" for the units that are displayed by the xScope tools. When points are selected, all measurements are displayed with a decimal point. Pixel measurements never show a decimal point. When space permits, "pt" or "px" is also displayed.
 

It may sound simple, but under the surface there are a lot of subtle details. To read more about these differences, open xScope's in-app help by selecting Help -> xScope Help. Then select "Using xScope on a Retina Display? Read our helpful guide" at the bottom of the help's home screen. There you can learn how the screen is scaled with different Display settings, how xScope shows the alignment of single pixels, how there are issues with precise mouse movement, and how the tools handle multiple display resolutions.