The ROI (Both SQM and ADU) defines a region of interest for two purposes:

SQM - This is the region where stars are detected and sky quality is calculated.
ADU - This is the region which is used to measure the amount of exposure and to modify the exposure time for future exposures to reach the requested ADU size.

x1,y1 are the co-ordinates of the top-left (with 0,0 being the top-left of the image)
x2,y2 are the co-ordinates of the bottom-right (with 1280,960 on the ASI120MC being the bottom-right of the image).

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