I also use one in the topoff reservoir. They are the same sensors as in the ATK and the DDR, so I keep some as spares for those devices. I also use one as a 'Skimmer Cup Full' sensor. Honestly that last is probably the second most useful (after the low level ATK sensor), although some would recommend a float valve in the skimmer cup given that you're working with gunk and foam and not clean water (OS subject to false CLOSED state), but with my close monitoring, I'm ok with that particular risk. The third most useful, although it's never been activated, is one that I mounted at the very top of the overflow compartment. It is designed to close and alert me if the water level rises that high, meaning that even the emergency drain has been overwhelmed for some reason.