Class vs job?

[Naria]

The job crystals get mentioned in quests quite frequently (mostly job quests) so no they are not just a gameplay mechanic. Jobs from the expansions start out as jobs and the reason why is touched on in those job quests. Lore wise job crystals contain the impressions of skills and memories of their previous wielders and therefor function as sort of a cheat to help people master complex skills quickly. That said the job crystal doesn't enable you to use all the skills in of itself (lore-wise). The wielder still has to practice and refine their own skills to master a job. This is why the ARR jobs go from class->job as by learning the prerequisite class you now have a basis from which to understand and build on the skills and memories granted to you by a job crystal. To unlock expansion jobs you have to have at least one job at 50 so they storyline just kinda of handwave the lack of class based off of your previous experience in combat and in using job crystals. There are only two jobs in the game where you start off with a blank job crystal, Blue Mage and Machinist.