This is the syslog severity level, from emergency down to debug, selected by a given log-forwarding filter, defaulting to debug. This matters because forwarding at too low a severity threshold, like the default debug, can flood a collector with noise, while too high a threshold might filter out messages an admin actually needs to investigate an incident. For example, an admin forwarding logs to a lightly resourced backup collector might raise logFacilityLogSelectorSeverity from the default debug to warning to avoid overwhelming it with routine informational messages.