From the introduction:
Welcome to the playtest for Pathfinder RPG Ultimate Combat, a new sourcebook for the Pathfinder Roleplaying Game. This book is set to include a host of options for every character that picks up a sword, mace, or bow to fight off murderous monsters, as well as three new alternate classes: the gunslinger, the ninja, and the samurai. Each alternate class replaces the class features from another class. When a character takes a level in one of these classes, he cannot later take levels in the other version of that class. For example, when a character takes levels of ninja, he cannot later take levels of rogue. For the purposes of prerequisites or other effects, each of these alternate classes counts as the class that it is related to.
Take these classes and use them in your game. Allow players to experiment with the rules in this document and use these classes against them as NPCs and villains. Are the classes balanced against the other characters in the game? How about against characters of the alternate class? Once you’ve had a chance to play with these rules, post your thoughts and feedback on the messageboards at paizo.com. Each class has its own messageboard for discussion and critique. The design team looks forward to hearing your thoughts. See you on the boards.