There are four main Classes in the game. These are unique and will have restrictions when it comes to Re-Classing through Seals. For example, Azura's default class, the Songstress, can't be acquired by anyone, including the main character. She also can't return to being a Songstress once she changes her class. The avatar's default class is also unique as only they can have access to it. Hoshido/Nohr Noble is only available to the main character and Kana.
Take note that Special Classes like the Songstress and Maid/Butler doesn't have advanced Classes so their level cap is LV40 and they can learn 4 skills along the way, similar to how base units promoted to advanced classes learn their skills.
* Hoshidan/ Nohr Noble will be exclusive on the route you're playing. In Revelations, you can choose from either of them.