@DavidCarlisle, oh, I see. So if \the\zOuter is expanded to \zInner while \zInner can farther be expanded, then \edef doesn't expand \zInner? So \the acts as expand once?
As for space and equals sign being optional, some assignments can't be done without them. Doesn't it imply they're mandatory in those assignments (but optional in a sense that not all assignments require them, e.g. \count1\count255 needs no space or equals sign).