In this sample compton.conf (from https://github.com/chjj/compton/blob...on.sample.conf), there is line #43 which has this content:
# opacity-rule = [ "80:class_g = 'URxvt'" ];
Can someone please explain what this line does when uncommented? Does it set an opacity of 80% for windows of URxvt? But then what is :class_g?