What does xxxHolic have to do with Tsubasa: Resevoir Chronicles?

I was looking up xxxHolic because i hace watched all of Tsubasa and i kept hearing things about it from other people who had watched it, but when i looked it up and was reading the plot line it explained about kimihiro and yuko but then it started talking about tsubasa: "During the climax of Tsubasa: Reservoir Chronicle, Yūko sacrifices her life to help Syaoran and Sakura achieve their goals. In order to both stabilize the shop's existence and break free of Fei Wong Reed's trap with Syaoran, Watanuki offers up the remainder of his existence and becomes the successor to the shop until the day he can grant Yūko's wish." Thats an exact quote. I know that CLAMP reuses their characters, thats why Yuko and everybody is in tsubasa, and i think syoaran is in an episode of xxxHolic but im not sure but they are completly different story lines, but anyways i was trying to figure out why it had what happened in tsubasa listed under the plot line of xxxHolic, like does a CLAMP make their series interchange or something, does one affect the other?