That is to say, "non-projection overlapping point" is replaced by "non-projection overlapping area". The condition is that there must be a projection area first, and if there is no projection area, the integral is 0; If there are projection areas, it must be ensured that the projection areas do not overlap.
For example, take the edge of a cylindrical surface perpendicular to the XOY plane, and if it is calculated as ∫∫Pdxdy+Qdydz. For the former, its projection on the XOY plane becomes a circular straight line, so there is no double integral, that is, there is no area, so the curved area is divided into 0. . . For the latter, when it is projected on the YOZ plane, the projections of the front and back sides of the cylindrical surface overlap, which is equivalent to invisibly calculating a part, so it has to be divided into the front and back sides for calculation.
It may not be so smooth, but it's all my own opinion. But if you deduce the integral formula of surface integral yourself, you may know why. . . . . .