Leveling Up is a process in which your Current Experience exceeds the Required Experience for the next Level. When this occurs, your XP bar will reset to zero to show that you have an entire bar to go to get to the next level. You will also glow, and show a 'Level Up' icon above your head.
In Tabula Rasa, when users reach the required Experience Point level for Levels 5, 15 and 30, the user must see a trainer to advance in Tier to continue leveling up. Although XP is accrued during this time, users will not gain the benefits of leveling up until they visit a trainer to change Tier Levels. Trainers are usually available in large towns/bases of the areas in which hunting is the best, such as Alia Das for Tier 2 and Tier 3 Trainers.
When you reach a Tier (Lv 5, 15 and 30), upon training for your next class upgrade you will receive two bonus skill points. Just before the training you'll also recieve a Clone Credit for you to be able to try both classes.