I will try to explain in detail how it works.
A character can have several classes. When reaching a certain level in the first class he is able to select a second class, then a third and so on. To increase a level in a class, he must have at least a specific number of skill-like abilities of at least a certain level, where at least one of those must be from among those that became available when he gained the last level in that class.
Example:
To reach level 4 in a class, he must have at least 7 abilities at level 4 where one of those must be one he gained at level 3.
In this system, the experience awarded is used to increase the ability levels, not to determine a class level, and the cost increases based on which level the class got access to the ability.
Example:
A character with a warrior type class would get the Melee Weapon ability at level one, and would then have to pay 100 xp for the first level.
A mage type character would perhaps learn that ability at level 9, and would then have to pay 300 xp for that first level.
A character with mage as a second class would have to pay 500 xp for the ability, and for a warrior second class it would cost 200 xp.
Then we could have a character with his first class being a mage, and his fourth class being a warrior (if we ignore those in between), that character would pay 300 xp for his first level in Melee Weapons from his mage class, or 500 xp from his warrior class.
Any ability obtained from his first class would automatically count against his needed abilities in his other classes, and so forth.
Thx