To be clear, while jobs gain levels through exp, the only way for jobs to get more job abilities (which have traditionally been gained via leveling up) is through job quests. So it's fair to say that jobs functionally "level up" through quests instead of exp.
While on the subject of job vs. class: with the lone exception of SCH, you should not use a job on any event levelcapped below 30.
A level 25 class gets 5 cross-class abilities (from any class).
A level 25 job gets 2 cross-class abilities (restricted to 2 specific classes), zero job abilities, and a small bonus to base stats.
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