In xxxHolic and Tsubasa, Yuuko says that there are certain things that she can do within her sphere of allowable interference such as
sending the original Syaoran to where the clone and the rest of them are.
This seems to only be related to the battle between Fei Wang and the main characters. Who set this sphere of allowable interference? What are its rules?
This is pure speculations since I can't find any reliable source, but I think this has something to do with a magician's limitation of powers. Like for example, Yuuko can only grant one's wish if that person is willing to lose something important to him/her. This quote is somehow relevant to this:
"To gain, something of equal value must be lost." - First Law of Equivalent Exchange in Alchemy (Fullmetal Alchemist)
If her powers has no boundary, then she should have grant any wishes without paying any price. And also, Clow Reed is a good example. He's the strongest wizard known in the Tsubasa Chronicle (Cardcaptor Sakura and XXXholic) universe and yet he can't revive someone who had already died (who was Yuuko) and was one of the reason for the conflict of Tsubasa Chronicle and XXXholic plot. Yet another example of a person's power limitation.
As to who set it, I really don't know. But this is probably a rule set by the Clamp writers.
This seems to be more like a law of nature. At some point, she has a conversation:
"A witch operates by many restrictions, doesn't she?"
"It must be, or everything comes apart".
I'm quoting from the top of my head - can't find the reference right now. I'm not quite sure about the second (Yuko's) line, but she made it clear that if she didn't abide by the restrictions, the consequences would be bad indeed.
So she is setting most, if not all, of the rules herself. Not because she wants to, but to avoid causing damage.
(It was in the manga, I think in Tsubasa, that she had this conversation. Will try to find the exact reference later).
