When dealing with any CAD program, it is vital to know what system of units you are currently working in. Without a system of units it would be impossible to know how large the object is that you are designing. What is the estimated weight of the object? What is the estimated inertia about the drive axis? Is the design within a balance specification? These questions would be difficult, if not impossible, to answer without knowing the unit system in the current part. As such, unit systems are very important when modeling.