There are four core classes in Scarlet Horizons. Each core class has two or more subclasses.
Upon attaining level 3 in any core class, you may, upon your next level advancement, (a) choose a subclass of that core class, (b) take level 1 in a new core class in which you heretofore had no levels, or (c) continue in that same core class by taking level 4 in it.