Garmin GTN 650 Series Pilot's Manual Upgrade Supplement page 127

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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GTN 650/750 Series SW V3.00 Upgrade Supplement 190- 01007-05 Rev A