In order to be able to draw on a three-dimensional model on different walls, or on sloping surfaces, AutoCAD makes use of changeable coordinate systems, called the "User Coordinate System" or the "UCS" for short. If you are drawing a floor plan, you will use the standard coordinate system, which assumes that you are looking down along the Z-direction toward the center of the earth, that the positive X-direction is to the east, and the positive Y-direction is to the north, and the positive Z direction is straight up (out of the screen). This is called the "WORLD" coordinate system. In Model Space there is an L-shaped arrow normally seen in the lower left side of the screen which shows the positive X and Y directions. This object is called the "UCS ICON." It can be turned on or off at will by typing: UCSICON<RET>ON (or OFF as the case might be). It will not plot.
The World Coordinate system works fine for any two-dimensional object, but when you draw a 3-dimensional building with a gable roof, for instance, you cannot draw on any of the elevations or on the roof without changing the orientation of the coordinate system. In a sense, you are always drawing on a "plan" of the portion of the building, whether it is a plan or elevation or roof. That is, you are always drawing on the XY plane of whatever coordinate system is current. To draw on an elevation, you have to define the XY plane to be parallel to the elevation on which you are drawing. Every User Coordinate System (UCS) that you create on which to draw may be saved and given a name so that you can easily return to it to draw more on that plane.
One of the exercises in this book will take you through the process of defining, saving, and drawing on several "User Coordinate Systems."