Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 364)

Coordinate Control
Chapter 13
13-12
Important: Any axis word in a block with plane select G-codes (G17,
G18, G19) causes axis motion on that axis. If no value is specified with
that axis word, the control assumes a value of zero or generates an error
depending on how your system is AMPed.
There are two methods for programming axis positioning commands,
absolute positioning and incremental positioning.
In the absolute mode, coordinates are referenced from the zero point of the
active coordinate system. Absolute mode is established by programming a
G90.
G90X40.Z20.;
In the above block the control will move the axes to a position X40, Z20 as
referenced on the active coordinate system.
G90 is a modal G code and remains active until cancelled by a G91.
In the incremental mode, coordinates are referenced from the current axis
position. Incremental mode is established by programming a G91.
G91X40.Z20.;
In the above block the control will move the cutting tool a distance of 40
units on the X axis and 20 units on the Y axis away from the current axis
position.
G91 is a modal G code and remains active until cancelled by a G90.
13.3
Absolute/Incremental Modes
(G90, G91)