After some research I found out that;
The facts :-
Euro's Off-Road rules used to state that the MAXIMUM artificial surface is 80%. (Not 10% artificial).
This rule was changed in 2002 because it could preclude some good venues.
In fact it was the Tiverton track in 2002 that prompted the change, because sand mixed with horse sh-t was not considered NATURAL by some persons.
This is the rule change that I didn't know the wording, hope this helps
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