You can understand that x and y can take any value, so they can be represented by x and y, but according to the discrete book, the renaming rule is used. If we want to understand it further, it is the problem of free variables and constrained variables. The reason why y is sometimes written and x is sometimes written is that people can understand whether it is a free variables or a constraint variable in operation. You should understand that X and Y represent free variables or constraint variables in different situations. For example, if your first formula is sometimes written as (ⅵX)(-P(X)VQ(X))- meaning no, it means that X is regarded as a constraint variable.

In some places, it was renamed as 丠丠丠 y (-P(X) VQ (y))-representing the denial that the constraint variables in p (x) and Q(Y) are different, one is x and the other is y, so it is understandable to change the name. Call me again if you don't understand. I hope it helps you.