I stumbled upon this parameter recently, and I really like it. Although I am not sure what's the logic behind it.
Active Radius for regular lights like point light defines the actual influence radius of this light and crops everything above. The situation with the Dome Light is not clear, when I adjust the Active Radius for HDRI map basically I can see that the lower values 'cause more GI into the scene or something? I have already found a great usage for this parameter, the scene isn't that dark with Active Radius at 5-25 and I can get rid of some very dark shadows that Karma creates which I don't really like most of the time (I think Karma's GI isn't that great most of the time, especially in dark scenes), but I am still not sure if the parameter was intended to be used like this?