An auxiliary coordinate system (ACS) is a coordinate system with an orientation, and/or an origin, different from those of the DGN file coordinates (the Global system). Although not exclusively a 3D concept, an ACS is most useful in 3D modeling as a drawing aid.

By using an ACS that corresponds to the location and orientation of a particular element, you can enter data points relative to parts of the model rather than the global origin.

Although using ACSs may seem complicated, AccuDraw uses them as a basic part of its 3D functionality, permanently storing arbitrary rotations. It is not necessary to learn much about ACSs to use them effectively with AccuDraw.