Levelup bonus is an automatic permanent bonus to specific stats, usually basic stats of a character such as hitpoints or global chance-to-hit. Levelup bonus applies when character reaches the specific level and cannot be removed by level loss.
Major bonuses: Character reaching levels of series(e.g. 10,20,30) might gain a rare bonus to stats(e.g. boost to some secondary stat) each 10 levels.

Unlike levelup points(extra stat/skill points), player has no control over it.