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.
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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".