The scene unit, also known as the display unit, is the basis of the system used to display measurements in 3ds Max. This value in relation to the scene is determined by both the Units Setup setting and the System Unit setting.
By default, the scene unit is Generic Units, which means that the measurements appear as numbers only in the interface, and the System Unit scale is 1.0 inch. Using this setup, adjacent home-grid lines are 10.0 units apart. However, if, for example, you change the Display Unit Scale setting to Metric 
Meters without changing the System Unit setting the same distance displays as 0.254m. Using any Display Unit Scale setting other than Generic always shows the display unit or units as part of the measured value; for example, 18.2.
If you examine the following illustrations and read the captions, you can see why it’s important for all artists working on the same scene to be using the same system unit, and further why it’s advisable not to change the system unit in the middle of a project.
A cubic box that occupies one home-grid square measures 10x10x10 generic units using the default measuring system.
The same box with Display Unit Scale set to Metric Meters measures 0.254 meter cubed. This box is the same size as the preceding one (10 inches=0.254m), but is being measured using a different system.
The same box with Display Unit Scale set to Metric Meters and System Unit Scale set to 1.0 Centimeters measures 0.1 meter (10cm) cubed. The size of this box is different from the preceding boxes because the scene uses a different system unit.