1. If my memory serves me well, symmetrycal balance is the equal distribution of visual weight on either side of an imaginary axis through the center of a design. If you dissect a composition in half on its vertical axis both halves would correspond to one another, like a mirror image of one to another.
2. It would be easier if there were some option to choose. I'm not completely sure, but in my view this statement should be completed like this: A building may achieve this design principle, even when it is not symmetrical, if it displays a sense of three-dimensionality. This sense helps to represent your work in three-dimensional view but on 2D surface.
