"You are when it comes from the fal'Cie," Lightning huffs. If this guy really doesn't know his Focus, he's gonna go cie'th, whether he likes it or not. They've got a ticking time bomb and that's not good.

"Fal'Cie brand humans as l'Cie and give them a Focus," she continues, opting for revisiting an explanation that literally everyone should know by now. Better to annoy him with that than to let even the chance that he might actually have amnesia like Fang slip by. "It's a vision, annoyingly vague. If you complete it before your brand turns, whatever it is, you turn to crystal. If you don't, you go cie'th and there's no coming back from that."

Vanille or Snow should probably be giving this explanation; Lightning really doesn't have it in her to be gentle about it.