I/J Mode: You can set the interpretation to be placed on I & J in arc moves. This is provided for compatibility
with different CAM post-processors and to emulate other machine controllers. In Inc IJ
mode, I and J (the center point) are interpreted as relative to the starting point of a center format arc.
This is compatible with NIST EMC. In Absolute IJ mode, I and J are the coordinates of the center in
the current coordinate system (i.e. after application of work, tool and G92 offsets). If circles always
fail to display or to cut properly (especially obvious by them being too big if they are far from the origin),
then the IJ mode is not compatible with your part program. An error in this setting is the most
frequent cause of questions from users when trying to cut circles.
from Mach3 Manual capter 5-36