G90.1, G91.1: Setting the Absolute/Relative Coordinate Mode for the Center of the Arc

G90.1 – absolute positioning mode for setting offsets I, J, K. When mode G90.1 is in effect, parameters I and J for the XY plane or parameters J and K for the XZ plane must be specified with the G2/G3 command.

G91.1 – incremental positioning mode for setting offsets I, J, K. This mode is valid by default.