Advanced Classes are optional classes characters can take upon meeting the prerequisites, effectively multiclassing with these “new” classes to gain their class abilities, HP bonuses, and effects. These additional classes go up to 10 levels unlike normal classes, and are usually more specialized in terms of purpose compared to normal classes. Unlike archetypes these options are available for anyone who meets the prerequisites, and some Advanced Classes may meld well with the archetypes a character may possess.
Level Restriction: Each Advanced Class has a level restriction (between 5th level to 10th level), and characters must be at that level or higher in order to meet the level prerequisite for the Advanced Class.
Spellcasting: Some but not all Advanced Classes include spellcasting progression. If the casting type is the same as the casting type of a class you already possess, the levels stack for determining progression. If you did not possess a casting type or the ability to cast spells previously, the class instead grants it to you as if you were multiclassing into a class with spellcasting normally.
Attacks per Level: Instead of the regular progression, it instead lists a bonus amount of attacks you receive per level. So if you are a level 10 Soldier you have 3 attacks per level, and adding 5 levels of a class that grants +2 attacks per level increases your total to 5 attacks per level.
Weapon and Armor Proficiency: This is gained upon taking the class as if you had multiclassed into a class which grants you more proficiencies.
HP: Advanced Classes do not gain starting HP, and instead only receive bonus HP for leveling up. This is due to characters being unable to start off from level 1 as the Advanced Class.
Multiple Advanced Classes: You can take as many Advanced Classes as you want, so long as you meet the prerequisites for all of the classes you’re applying for, and none of the class requirements conflict with each other. For example, if a class requires you to not be able to cast magic, it restricts you from taking levels in Mage and so on.