The Interconnected Universe of CLAMP
The characters and settings in CLAMP's works often interconnect with each other.
When the members of CLAMP were still active on the doujinshi (self-published manga) scene before their commercial debut, they would play a sort of game by individually coming up with settings and characters and then placing them in a shared universe. They made use of that experience their early works known as the "CLAMP SCHOOL Trilogy." The series is set in the same school, with the various characters making guest appearances outside their respective stories. Other patterns that hint that the worlds are interconnected include TOKYO BABYLON/X and Wish/Kobato. crossovers, in which the story continues in another title, and ANGELIC LAYER/Chobits and SUKI/LEGAL DRUG crossovers, which share some of the characters.
As you can see, crossovers are a common occurrence in CLAMP's works. Their most notable title that features the most characters from their past titles up to date is Tsubasa -RESERVoir CHRoNiCLE-, which can be described as the culmination of CLAMP's universe. The characters are alternative versions of themselves that are removed from the context of the original story. This technique is referred to as the "star system," a nod to Osamu Tezuka, who treated his characters like actors. Such recurring use of characters brings happiness to the readers who are familiar with their past works. It is only made possible thanks to CLAMP's large number of published titles.