A player class is a type of actor meant to be player-controlled. They must be derived from PlayerPawn. Each supported game features its own player class, and Hexen as well as some mods such as TUTNT offer you a choice of player classes when starting a new game.
Because the choice of player class is made by the player, rather than by the map, it is not possible to use the "replaces" keyword to change the class list available when starting a game. Creating new player classes therefore cannot be done purely with DECORATE if the classes are to be made available in the class selection menu.