The order in which holidays occur in a diary, for a given token is based on 2 levels of ordering.
In the above example, you are garanteed that, for a given date, all holidays will occur in your diary in the order of the sets listed. So for example, if on October 12, there was a holiday in both the USA only and Canada only lists, then the holiday from the USA only set would occur first.
In the above example, if 2 holidays rules of the USA only set occur on the same date, then the holiday rules will appear in the order of their relative priorities (going from 1-20). The relative priority only has an effect for holidays and one-time-dates within the same Holidays Set.
For more details, see : The order of Holidays.