Your character's rating is the mathematical sum of all your adventure schools' level as follows:
Rating = square root of (
the sum of (
(current adventure level) power of 2
+ [(adventure school 2 level) divided by 2] power of 2
+ [(adventure school 3 level) divided by 2] power of 2
+ [(adventure school ... level) divided by 2] power of 2
)
)
The rating is rounded down to the nearest whole number. An online rating calculator ([1]) is available to check your rating and plan your character.
Example: a current Storm Disciple has a level of 40, with 20 levels in Druid, 20 levels in Warrior, and 20 levels in Monk. Their rating calculation would be:
square root of ( sum of ( 40^2 + [20/2]^2 + [20/2]^2 + [20/2]^2)) = square root of (1900) = 43 (rounded down).
Your adventure rating is used to determine the experience you get from defeating creatures in Istaria. In the example above, the Storm Disciple would get more experience when defeating foes that were equal to or above his/her rating than if they defeated a foe that was of a lower level than their rating. You do not get any experience for defeating a creature that is more than 20 levels below your rating.