For surface or line integrals, such as curves and surface integrals, all points to be integrated satisfy equations (corresponding surface equations and curve equations), so they can be replaced.

However, if the region is integrated, such as the region surrounded by the circle x2+y2=a2, then x2+y2 in the integration function cannot become a2, because the point in this region does not satisfy the equation, but x2+y2.

In short, all the points that are integrated satisfy the equation and can be replaced; Not all points satisfy the equation, so they cannot be replaced; What is common is the two situations in the above two paragraphs.