Advanced Classes are the branches related to your Main Class. As you increase your level, you’ll gain access to advanced classes. Advanced Classes unlock new skills that may improve or replace the ones you currently have. To help you decide which advanced class to take, you can tap the Details button to display the new skills and read their descriptions.
Your first (Tier I) advanced class can be chosen after reaching LV39 and once you’ve reached LV60, you’ll be able to upgrade to the Tier II advanced class. On top of reaching the required levels, you also have to complete the respective Class Advance Path quests to complete the access to the advanced class.
To make it easier to understand, let’s use my Miko as an example. Once you’ve reached LV39, Miko users can choose to become a Fan Dancer, Spirit Maiden, or Spell Master. For example I chose to become a Spirit Maiden, once I reached LV60, I will become a Priest. It’s important to note that the Tier II advanced class you’ll get will depend on the Tier I advanced class you selected. In the example above, once my Miko became a Spirit Maiden, the only Tier II advanced class at the end of the progression is becoming a Priest - not a Dragon Dance Master or Psychic Master, which are Tier II classes of the other branches.
If you change your mind, you can still change to another branch and start back at Tier I but that will consume an Advancement Reset Scroll. You’ll get one for free from one of the main quests. More can be bought for 500K gold each.