How do the 9999 and 99999 limits work?

[Tavrobel]

To break Weapon Damage Limit, you need the Break Damage Limit, which is synthesized from 60 Dark Matter, a drop from all of the original Creations as a Rare drop, ranging from 12.5% to 25%.

The Celestial Weapons also come equipped with this ability if I'm not mistaken

Those too, but I didn't feel the need to note it, since it would be made obvious after getting Yuna's Nirvana.