Zack breaks the HP limit when accessing a Limit Break or when leveling up. He can also break the HP limit when equipped with the Genji Armor or Adaman Bangle. The process is notably different in this game, however, because Zack can break his max HP limit right from the start of the game with a strong enough Limit Break or power surges from said Limit Break.
Break HP Limit is the second-most dominant ability for an armor, behind Ribbon. A notable occurrence in the names of the armor with the Break HP Limit is a "Genji" prefix, theoretically making them the Genji equipment of the game.
The Shield for Tidus with Break HP Limit is the Genji Shield. The Ring for Yuna is the Arcane Ring. The Bracer for Auron is the Genji Bracer. The Armguard for Wakka is the Super Goalie. The Armlet for Kimahri is the Genji Armlet. The Bangle for Lulu is the Minerva Bangle, following another Final Fantasy tradition as the Minerva Bustier. The Targe for Rikku is the Atlas.
When paired with Break MP Limit, it becomes the most dominant auto ability combination for an armor. With this combination, Tidus has the Endless Road, Yuna has the Solomon Ring, Auron has the Overlord, Wakka has the Indomitable, Kimahri has the Pride of the Ronso, Lulu has the Samantha Soul, and Rikku has the Dreadnought.
Characters can break the HP limit via the Enterprise accessory and through passive abilities available on the Higher Power and The End Garment Grids. Only the special dresspheres have Break HP Limit as a job ability, after the acquisition of certain key items.
Although breaking the 9,999 HP limit is not visible, it still occurs in the game and can be used to successfully damage enemies dealing up 9,999 points of damage. As enemies' damage output is capped at 9,999, if the party's HP exceeds that, they cannot be killed in one hit apart from Instant Death attacks. The only way to get HP high enough to go over the limit is by using Bubble.
Like in Final Fantasy XIII characters and monsters can have over 9,999 HP by default.
Lightning can reach 999,999 HP making her the first playable character to reach such a high HP level. This ability is exclusive to her and appears only in "Requiem of the Goddess" downloadable content scenario.
While characters could only achieve 9,999 HP in Dissidia Final Fantasy, this limit was lifted from the prequel and characters' HP can theoretically go up to 999,999.