A "multiverse" created by Tezuka Osamu involving character of his works making appearance in his other works, typically in other roles as if they were playing a role like a "real life actors". Many characters were reused from his other works, while some (such as Rock) were created specifically as a "superstar" who appeared in several works rather than having one of their own.
Description from Tezuka Osamu's official site
The "star system" employed in Tezuka Osamu's Manga differed slightly from its Hollywood namesake, with the characters who appear being handled just like movie actors. In other words, as one actor takes on a variety of different roles, so individual characters were given different parts to play.
Aside from the works of Tezuka Osamu, no other Manga artists were using this visionary formula in their writing. It was inconceivable that the star of one Manga would appear as a backseat player in another, and in the guise of the villain at that.
Nonetheless, Tezuka Osamu thought of his characters as real live actors and actresses and took great pleasure in his role as director in assigning roles to them. Readers who spotted Atom in the guise of the son of a kidnapper in another work would smile with delight.
Tezuka Osamu even went so far as to think up appearance fees for his characters and his Manga-making techniques overlapped with movie production methods in many ways.