When surface detection finish,
click the surface to add cube,
scroll to move selected cube,
click cube to change selection.
The bottom of the screen will show the distance of each cube.
Google Pixel, Google Pixel XL, Samsung Galaxy S8
Distance is calculated by the tx ty tz of two pose. In my experience, I use Google Pixel, sometimes the result of arcore is only 1~2cm different from the result of a ruler in real world, but sometimes two results will differ by more then 10cm.