Main story is the... main story of the game. It tells the story of how a wandering adventurer(you) showed up in gridania one day, and after many trials and tribulations went on to save the world repeatedly. So yeah, that goes from level 1-they stop making the game. You play as a class from level 1-30, so class quests. You play as a job from 30-the end, so job quests for that. If your goal is machinist, then bard(pugilist is no longer required) until you get inside the city of Ishgard(probly around level 50-52) which is where you can receive the training. Machinist starts at level 30, so youll have to take a short break from main story quests when you first unlock, but once you get caught back up you just continue onward using a gun instead of a bow.