Designer gives you exceptional flexibility in defining scale units without causing unwanted changes. Note that changing a unit's values in the Add Custom Unit dialog box does not automatically change the overall units definition for the rulers in the current drawing. To change the ruler unit settings, choose the setting again in the Rulers/Snap tab of the Options dialog box.
In addition, changing a unit's values in the Add Custom Unit dialog box does not automatically change the overall units definition for any other drawings using that unit. This is because the various units set for different functions are saved with each Designer drawing.
For example, suppose that the rulers are set to a custom scale unit called Meters (Scaled), which defines a scale of 5 meters per centimeter. You open the Tools Menu, click Custom Units, select the unit, click Edit Unit, and edit this scale unit by changing its scale to 10 meters per centimeter. The ruler unit settings do not change unless you open the Tools Menu, click Options and Rulers/Snap, and deliberately change the Ruler Unit to Meters (Scaled) again.