Axis is a line in a plan view, which can also be understood as an infinite surface perpendicular to the earth, with direction and position, or a vertical section. This "face" is perpendicular to the earth and passes through the axis. All the spaces on this face belong to this axis. Axis is valid at any height in space. This is a spatial concept, and this "line" is used from the first floor to the top floor. If the axis is understood as a point in the elevation, it really becomes a "line" of space, which has no practical significance. Of course, I can't understand that it has also crossed books. It describes the spatial location. It has nothing to do with the book itself. No matter how the book is placed, the icons on the picture will not change up and down. You study like this just to "drill an alley" and "come out" quickly.