The Doppeliner System is the system that maintains the balance of tera on Earth.

"Doppelgangers are just a myth made up by humans, what really exists are known as doppeliners. They are part of a system created to maintain the balance of tera in this world, there are three of the same person here on Earth and their fate is divided up amongst them; in accordance with the rules of the coexisting equilibrium, if two subs meet then they'll be extinguished via an illness, accident, or what have you. After that the remaining group will absorb the fate left behind by the extinguished subs."
KuroKurokamiEpisode 1

However, tera is not split equally and because of this they are divided into two groups. The one with the highest tera is called the root, and the other two are subs. But when two subs meet and their tera is absorbed by the remaining party, that remaining party becomes a master root, holding 100 percent of the tera. This is illistrated below.