Its like this..

WHM learns Poisona at lvl 6. So if you sub WHM, you can use Poisona at lvl 12. So the subjob is half of your main job, and you can use any magic and abilities your sub has along with your main job's. Equips however is a no. You wont find a Red Mage equipping a Lance even though it subs DRG unless the lance lets you.