Cross Classing is the means through which a player can take up an Advanced Class, without having the usual Trainer Class prerequisite. Taking a Class through Cross Classing does take up one of a player’s three Class slots, but provides you with a way to take multiple Advanced Classes from different Base Classes. Taking any class through the Cross Classing list does come with a penalty: You do not gain the Stat Bonus, usually gained through taking the Advanced Class and you only gain one of the Advanced Class’ starting Features . You may choose which of the two starting Features you start with, and you may gain the other starting Feature by taking it as you would any other Feature when leveling up (the starting Feature you do not take does not have any prerequisites).
Cross classing requirements are listed directly under requirements with the appropriate base class on Advanced Class pages.