Many different applications...
Last year fitted 100 amp Prestolite alternators to my aeroplane (the Westinghouse units removed suffered from poor parts availability and are/were difficult to balance) and the aux terminal is used for output fail indication, at least on this particular installation with transistorized voltage regulators.
My GPS unit fitted shows bus voltage, and the output from these alternator/regulator combination is very stable...28.0 volts precisely, adjustable as desired.