For a CNC lathe the G99 code word toggles the feedrate to inches per revolution as opposed to inches per minute with G98. If feed per revolution mode is ON then the F word in any block will be read in inches per revolution.

For example G99 G01 X1Z1 F.02 will feed the tool at .020 inches per revolution.

Compare this with G98 G01 X1Z1 F2 which will feed the tool at 2 inches per minute.