Say for example you wanted to avoid entering any area of London due to Congestion Charge or Low Emission Zone you could create a kml file with all the co-ordinates. You will be diverted away from this area as the road will be recorded as blocked so the gps will automatically reroute you to a more sensible rote thus avoiding the whole area.

Example:

Spoiler: Show Text
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.2">
<Document>
<name>avoid</name>
<Placemark>
<name>Hrdly_most</name>
<Polygon>
<outerBoundaryIs>
<LinearRing>
<coordinates>
0.26362,51.60784,0
0.26882,51.59899,0
0.27065,51.58980,0
0.27905,51.58139,0
0.28535,51.57264,0
0.28985,51.56408,0
0.29754,51.56004,0
0.30694,51.56343,0
0.32168,51.56471,0
</coordinates>
</LinearRing>
</outerBoundaryIs>
</Polygon>
</Placemark>
</Document>
</kml>


The coordinates in above example resolve like this:-

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All you need to do is create the polygon shape via another website and paste the data into the space between <coordinates> and </coordinates>

Once the kml has been created place it into folder ... \ content \ userdata \ Avoid (you will need to create a new folder called "avoid".