“External NMEA 2000 Alerts†means the NAVAlert will respond (sound an alarm) if it receives Alert PGNs from other devices on the network – rather than just its own Alerts.
“Miscellaneous NMEA2000 Warnings†means the NAVAlert will respond (sound an alarm) if it receives any other PGNs that have an alarm field – like the warning light conditions in the Engine PGNs.