It goes in a tree pretty much.
Theres Novice, Merchant and then either Blacksmith or Alchemist.
Upon reaching an Alchemist or Blacksmith, you then reach towards lvl 99, at which point you are then offered the opportunity to Transend, becoming a Transendant class.
At that point, it goes: High Novice, High merchant and then either Whitesmith/Mastersmith or Biochemist/Creator.
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Now if you're confused about extended classes and normal classes and transendant classes, then you can check out iro.wiki.org