Each point represents a star in or near an ACS/WFC observation (black outline). We normalize the magnitude of each star according to the exposure time in seconds:
mag500 = magnitude - 2.5 × log(exptime / 500)
The resulting mag500 is a measure of the total fluence, or flux multiplied by exposure time:
flux500 = 10 ^ (-0.4 × mag500) = flux × (exptime / 500)
The greater the fluence, the more charge deposited onto the CCD, and the greater the scattered light artifact for a given star position relative to the ACS/WFC detectors.