I don't think I understand the phrasing correctly but let me try explaining specific game points and maybe that's the answer you were looking for.
FFXIV as you have probably known by now, has a unique character system, you're able to level all professions including gathering/crafting on one character. With that, it's up to you which of your characters become your so called "main" and doesn't have any additional restrictions besides the level restriction itself. In easy words, if you unlocked machinist (MCH) you can play it at that level 30 and level it all the way to 60 (70 in upcoming expansion), it does not matter if your black mage (BLM) is only level 15/30/50/55/60 because it doesn't effect the actual machinist job by itself.
However, when switching back to your black mage as example, you would be only the level you stopped at. You're not automatically the same level as your machinist (that only works for the duo job aka smn/sch) and would have to level that job if you're interested in playing it.
The only time were it matters that you have a level 50 character already, and finished the ARR story line, is the moment you want to play Machinist (MCH), Astrologian (AST), Dark Knight (DRK) because these jobs are not unlocked at character creation and can only be unlocked in Ishgard. You won't be able to access the Heavensward expansion (HW) without having finished the orginial/base game (ARR). But you did that already.
With that out of the way, if you're staying on that one character (in your case your black mage lvl 50) and level all your jobs there, then there's no additional requirements/restrictions applied and you can "main" your machinist if you chose to do that. But if you're now making a new character on the same server/other server then this doesn't automatically unlock the new jobs on that alt/new character. You would have to go through the whole procedure again:
Level 1 character of war/magic to level 50
Finish the whole Final Fantasy Reborn base game
Enter Ishgard and pick up one of the new jobs that were only added with the expansion.
I hope that gives you some clarity or maybe other people can answer your question better.


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