Also undocumented in the source, but its name and read-write Gauge type suggest it controls the state of the router's first physical digital output pin, complementing the read-only input reported by ioBin0. An admin would write a value here to switch that output on or off, for example to trigger an external device like a siren or a relay-controlled lock. For example, writing a value of 1 to ioOut0 might energize that output line, physically closing an external relay wired to it.