Changing into a job increases stats and allows new abilities to be used. However, it limits the amount of role actions that a character can equip from ten to five, forcing them to pick and choose which abilities they would like more carefully. Thus, it sacrifices versatility for specialization.
What a Soul Crystal actually is is largely unexplored, but its stated purpose varies from job to job and it is consistently described as having a wealth of knowledge related to the job contained within it, containing an extreme concentration of aether, and being carved with the deeds and exploits of past people to hold the job's mantle. The sole exception to this lattermost rule is Machinist, a job completely new to the world of Final Fantasy XIV with a Soul Crystal described as completely pristine as a result.
Generally a magic-related job will attribute some importance of the crystal to their actual spellcasting, such as the crystal for Black Mage being necessary to prevent self-immolation from loss of control over spells (though it's also shown black magic is still possible without it, and practice and discipline still allows one to be an effective black mage), or the crystal for Summoner being a catalyst for the summoning of a primal's Egi. Physically-related jobs, such as Ninja, may describe certain techniques as being extremely difficult or impossible without their corresponding Soul Crystal, though some jobs will not offer an explanation for their Soul Crystal whatsoever. As well, Dark Knight is shown to at least be functional without its Soul Crystal, as it shatters at a point of the job's storyline (for gameplay purposes it is still whole and an equippable item) and needs to be repaired through absorption of the crystal's aether, but its state of disrepair has no adverse effect on the adventurer's techniques or capacity for them.
Some Soul Crystals may also contain the literal souls or consciousness of previous bearers as well, such as the Soul Crystal for White Mage containing the soul of A-Towa-Kant, the person to bear it before the Warrior of Light.
soul, in many religious, philosophical, psychological, and mythological traditions, is the incorporeal and, in many conceptions, immortal essence of a person, living thing, or object.The