Garmin GPS 400W,GNC 420W Pilot's Manual & Reference page 151

Note that the airspace alerts are based on three-
dimensional data (latitude, longitude and altitude) to
avoid nuisance alerts. The alert boundaries for con-
trolled airspace are also sectorized to provide complete
information on any nearby airspace. Once one of the
described conditions exists, the message annunciator
will flash, alerting you of an airspace message.
To view an airspace alert message:
Airspace Name
7 - NRST PAGES
Nearest Airspaces
141