Overview
The FFZ sub-job system allows a high-level character to add diversity to their ability selections.
Essentially, beginning at 6th level, you can start gaining abilities from the "job ability" of one other job. The job need not be related to you on the Job Tree: anybody can select any job for their sub-job.
You don't get any of the "base abilities" of that job: the job's traits, attacks, defenses, limit breaks, or stat bonuses. Your statistics (weapon power, magick power, stamina, willpower, etc.) don't change. You only get abilities that derive from the job ability of that job (for instance, Alchemy for the Alchemist). Your level 6 is considered "level 1" for this new job, and you gain abilities essentially as if your level in the sub-job was equal to your level minus 5 (for instance, at level 8, you gain level 3 abilities; at level 12 you gain level 7 abilities, etc.).
You retain all the features of your first job (sometimes called your "main job," "primary job," or "base job"). You don't have to take a sub-job, but you gain no benefit for passing it up — FFZ assumes every character over 6th level has a sub-job.
Sub-jobs don't generally enhance your character's power by a large degree — a level 1 ability at level 6 is generally not exceptionally potent, even if it was a fantastic ability at level 1. What they do is enhance your variety and adaptability. This makes them especially wonderful for narrowly focused primary jobs. More versatile base jobs (such as the Freelancer) may benefit from particular focus in one area, or may enhance their versatility even further by shoring up one of the few areas in which they lack (for instance, if a Freelancer were to gain more things to do with their weapon power). A sub-job can also enhance your focus, however — an alchemist gaining machinist abilities will be an expert user of tools, for instance, while a freelancer to takes up the Wizard sub-job will be a destructive spellcaster who has a well-rounded base.
Sub-jobs generally won't define you as strongly as your main job, mostly because they are weaker. They might give you an edge or allow you to exploit a strategy, but they generally won't alter the way your character plays. They are additional abilities, not replacement abilities.
