Zone

Zone is an ability possessed by initiators who have reached their maximum potential to go past their limits. Not much is known about "Zone."

Zone's Introduction
Doctor Sumire Muroto introduces the concept of Zone to Enju Aihara in the second volume of the Black Bullet light novel series as a solution to get past the metaphorical wall that Enju had reached in terms of abilities. However, Dr. Muroto warns Enju against trying to achieve it, as it would raise her already high corrosion rate. Enju disregards this advice, as she doesn't know her true corrosion rate, and tries to attain Zone anyways.

Zone Attributes
Zone is described as a way to break through the preset limits of an initiator's powers. If an initiator with a model rabbit factor like Enju Aihara were to attain it, they never cease to increase in both their speed and jumping abilities. If an initiator with a model dolphin factor like Kayo Senju, for example, was able to attain it, their intellect would continually increase past any human's and would likely reach incomprehensible levels.

How to Achieve Zone
Sumire states that to attain Zone, one must have control of both body and mind, and train relentlessly. She explains that once Zone is achieved, the initiator can never go back to not having it. However, it is extremely difficult to attain Zone, as evident from few initiators have done so. It is not very common knowledge either, so Sumire's description of how to attain it may not be entirely accurate. A by-product of attaining and utilizing Zone is the speed-up of one's corrosion rate, which is not dangerous in all cases, but would definitely be taken into account as a risk in cases such as that of Enju's.