Each class has a template describing both game and roleplaying signifigance. Custom core classes are, for the most part, classes or variants of core classes with no pre-requsites (i.e. anyone can start them from initial character roll, providing they meet race/stat requirements). Custom Prestiege classes exist to expand a core class in a specific direction, offering more flavor than a breand and butter version of a class. They generally have multiple mechanical and occasional roleplaying pre-requisites and restrictions, and can never be taken as a starting character.
Custom Core Classes are classes with some restrictions of race or mechanics, but can be taken by any player at initial roll.
Custom Prestige classes exist to expand a core class in a specific direction, offering more flavor than a bread and butter version of a class.