G90, G91: Setting the Absolute/Relative Coordinate Mode

G90 – absolute positioning mode. In absolute positioning mode, the axis parameters usually indicate the position in the active coordinate system.

G91 – incremental positioning mode. In incremental positioning mode, axis parameters usually indicate increments relative to the current position.

G90 example:

G90 (absolute position setting)
G0 X2.5 (fast movement to position X2.5, taking into account all offsets)

G91 example:

G91 (incremental positioning reference)
G0 X2.5 (fast movement of 2.5 along the X axis relative to the current position)