Choosing a New Class Tree: A new class tree is simple a new directipon for your character. A fighter may decide to study magic and open up the magus arcanum / warrior class tree. So while two proeliators may start out as near equals at 21st level one could chose to become a hybrid angelus spirit and proeliator and another could choose to become a shadow warrior/proeliator. In this way classes become more specialized as you move along and characters may evolve as their character gains more levels.