The Enhance system allows for players to level up any form of equipment, though the methods of doing so for each equipment type varies. Normal upgrading will add EXP to the item, leveling it up and increasing its stats, which increases the overall battle potential of the party if the upgraded item is equipped. Limit Breaking increases the maximum level of the item, but does not increase its stats - it merely increases the maximum degree to which stats can be added to it.
Any of the Enhance mechanics below that allocate EXP will have a base Great Success rate of 10%. Should Great Success occur, the amount of EXP accrued will be doubled. Great Success has no effect on any of the other mechanics detailed below which do not deal with EXP.