It is based on the rating of the horse. This makes it beneficial to older horse allowing them to be able to race for longer. As in racing in a "class" race refers to how many wins you can have, therefore if racing in a class 2 you cannot have more than 2 wins. In the ratings based system a horse can race in a race depending on their current rating (e.g. if a horse won 8 races but is now a 7yo and has not won for a while they can drop back in class. As they have 8 wins they under the class system must race in open races, which may be too strong. So if they are not performing their rating will drop and can then race in the rating equivilent of a class 4).

0-68 is the lowest I believe and then goes up from there. It can be quite confusing but is very good and has been worknig in other contrie for quite some time.