This command is used to enable Constant Surface Speed (CSS) mode in lathe version.
G96 followed by D & S values.
D Value - This is to set the maximum spindle RPM that the spindle can achieve in CSS mode as the tool gets closer to the center.
S Value - To set surface speed value. The units will be the current machine units in use.
N10 G96 D2000 S200
In the above example CSS mode is enabled with a surface speed of 200 and the maximum RPM is set to 2000.