Lets you enable rotation or scaling of one or more objects around their collective geometric center.
The Use Selection Center button is available from the Use Center flyout.
If you transform multiple objects, 3ds Max calculates the average geometric center of all the objects and uses that for the transform center.
The axis tripod shows the center that is currently being used.
Note: The transformation center mode is set on a transform-by-transform basis, so select the transform before you select the center mode. If you do not want the center setting to change, turn on Customize menu 
Preferences General tab Reference Coordinate System group Constant.
Preferences General tab Reference Coordinate System group Constant.
With the Selection Center option, an averaged coordinate system is used to rotate the objects.