Avidyne IFD540 Pilot's Manual page 91

At large map ranges beyond 250nm from the aircraft, small areas
of high-intensity RADAR returns may not be displayed; instead,
larger areas of surrounding lower-intensity RADAR returns will be
shown.
Diagonal stripes show the Datalink radar data coverage area. In
normal operation the boundary follows the outline of the
Continental United States (CONUS). If, however, Datalink radar is
unavailable in a particular area for any reason, hatched lines
appear in that area. In the mountains and off the coast, hatched
lines may represent no coverage below 10,000 feet. If there are
radar returns in that region above 10,000 feet, the returns will be
displayed as "islands of precipitation" surrounded by the hatched
lines.
No Datalink Coverage Area Hatched Lines
Map Pages 3-15