The Units in CSS are either Meters or Feet depending on whether your code is metric or Imperial.
So if metric the S word after a G96 will be meters, so G96S200 will be a surface speed of 200m/min.
In your previous post you said that the site you had looked at was saying G96 was constant RPM, it is not. G97 is constant RPM, G96 is constant surface speed.
If you call G97S200 it will rotate the spindle at 200rpm and it will not vary if you vary the X axis and it will not matter what the current X axis value is when you call the G97S200.
If you call G96S200 it will start the spindle rotating, what the RPM will be will be dependent on the current X axis position. As the X axis moves the RPM will follow to keep the surface speed constant, so decrease the dia and the spindle speed will increase, Increase the Dia and it will slow..
Hood