It’s almost like every book in the DC universe was told to expand their mythos. The Batman family got a new member, so did the Wonder Woman family. Superman had the Phantom Zone added. And the JLA got a new member. So what about Green Lantern? Well, Green Lantern #6 (March 1961) added the concept of multiple Green Lanterns patrolling different sectors of space, and Hal Jordan meets Tomar-Re, the Green Lantern of sector 2813.
Tomar-Re’s home world is threatened by invading space monsters at the same time there is a crisis on another world in his sector. Tomar-Re contacts the nearest GL, who happens to be Hal Jordan. After the adventure is over, Hal Jordan meets up with Tomar-Re to gain some exposition, er, knowledge.
For example, he learns that the Guardians of the Universe are responsible for the power batteries and certain Green Lantern missions. He also learns that all power batteries charge rings over the same time frame (roughly 24 hours of Earth time).