Answer:

This is referring to the angular deviation between a given ray and the chief ray, in image space. In afocal mode, the idea is that the output wavefront is planar (so, the output ray bundle is perfectly collimated). This means that there's ideally zero angular deviation between any two rays in image space. OpticStudio uses the chief ray angle as the (0, 0) reference angular coordinate (regardless of it's image space direction cosines). Then, sampling across the pupil, it checks angle each ray makes with respect to the chief ray, breaking the angle into the X and Y components. If there's any angular deviation in either the X or Y directions with respect to the chief ray, that shows up in the angle-space spot diagram as an offset from (0, 0) in the X/Y direction respectively.