When 3D-based files are first created (in MicroStation, for example), working units are defined. Working units are coordinate units that correspond to the position and definition of a point in a design file. These coordinates appear in the status bar as you work in Smart Review. There are a fixed number of points on each axis of a design file. The idea in establishing working units is to make sure that the objects that you want to draw can be entered with adequate precision, and at the same time fit in the design plane.
Working units include master units (the largest unit in use in a design), sub-units (a portion of a master unit), and positional units (a portion of a sub-unit). Positional units are the smallest degree of accuracy possible in your design. Working units are usually referred to as MU:SU:PU. Most models in Smart Review use feet or meters as master units, inches or millimeters as sub-units, and thousandths of an inch as positional units.
For example, in a plant with MU=feet, SU=inches, and PU=.001 inch (one thousandth), an 8 3/4-foot pipe is measured as 8:9:0.