Well, some worlds have things called 'flaws', where something is wrong within the world, like... within it's programming I guess is a good way to look at it?

[ It's the closest thing she can come up with, considering that she never really used computers before. ]

When someone arrives in the city, they're considered Wakened. After some time, Nautilus chooses some people to Ascend, sort of moving up a level. The level above Wakened is called Angelii. Your Bending ability gets stronger and you get a lot of other abilities as well, such as being able to sense flaws. Angelii usually work in tandem with the next level, Ashura, in order to find flaws. Angelii sense them, lead the Ashura to them and then the Ashura fix them.

I became an Angelii almost nine months ago, both Brother and Suzaku Ascended to Ashura before I Wakened.