G97 is the default spindle mode for the control.
In this mode the S value will set the spindle speed in Revolutionss Per Minute (RPM). The speed will stay at this value until it is changed.
If the spindle is turned off and then back on in the program the speed will still be the previously set value.
This mode is good for drilling and fixed spindle speed operations.
To set the control so that the spindle speed is in Surface Feet per Minute (SFM) see G96 Spindle Speed as SFM.