(Use Selection Center), on Use Center flyout
The Use Selection Center button, available from the Use Center flyout, lets you enable rotation or scaling of one or more objects around their collective geometric center. 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.
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
Reference Coordinate System group
With the Selection Center option, an averaged coordinate system is used to rotate the objects.