In Japanese the term Genkai means Limit or Bound.  Final Fantasy XI has several level caps. 

You must complete certain quests to raise these caps.  You must do one at 50, 55, 60, 65, and 70. 

If you don’t complete the quests, you will still hit level 50, but you will stop gaining xp at 1xp till next level. 

The final level cap is 75 and there is no quest to go beyond 75.