You can USE abilities from other classes that you have leveled up. This is where you select which abilities you want to use and currently you can select a maximum of 3 which will raise to 10 when you are level 50. If you are on a job then you can only use 5 abilities from 3 specific classes based on which job you are on. So to get more abilities you need to switch class and level it up until it has a learned an ability that can be cross classes (not all of them can ve used on other classes) then you can select this ability on your other classes.