Well its kinda complicated...
But on the other hand really easy..
When u become Bowman for example you can choose between two other classes when u reach lvl 10... after that it will advance every 10 levels...
But you can't choosse then.. .so..
Bowman... then u choose woodsman... than u can choosse Hunter..
then u automaticly become ravager and amazon...
so its not too hard... you don't really have to break ur brain on it.
(Sorry when my english is bad
)