The value format is based on IIIF standard, pct corresponds to percent (so it it's possible to change the image file with an image at the same ratio but not the same dimensions in pixels) and w,x,y,z numbers delimit a rectangle. Example for Idleness (Q19953492), with depicts (P180) qualifier house cat (Q146) value "pct:65,81,35,15" → https://tools.wmflabs.org/zoomviewer/proxy.php?iiif=Godward_Idleness_1900.jpg/pct:65,81,35,15/,240/0/default.jpg .
A specific tool on Crotos IIIF Cropper can be used to get the adapted values. In the same tool, it's possible to display the result ; for example : depicted items in Ancient Rome (Q14619165). --Shonagon (talk) 01:18, 22 November 2016 (UTC)
Second property needed
I just encountered this use of the property referring to a specific image file. As pointed out in the proposal discussion if we want to provide this functionality we need another property for that. --Marsupium (talk) 15:48, 19 September 2017 (UTC)