Garmin GTN 725 Cockpit Reference Manual page 101

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NOTE 1: There are several reference datums that waypoints can be
surveyed against. TSO-C146 normally requires that all waypoints be
referenced to the WGS84 datum, but allows for navigation
to waypoints that are not referenced to the WGS84 datum so
long as the pilot is notified. Certain waypoints in the navigation
database are not referenced to the WGS84 datum, or their reference
datum is unknown. If this is the case, this message is displayed.
Garmin cannot determine exactly how close the non-WGS84
referenced waypoint will be to the WGS84 datum that the GTN uses.
Typically, the distance is within two nautical miles. The majority of
non-WGS84 waypoints are located outside of the United States.
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