Hi Per,
Not that it is of any consequence but the I and J are the X axis and Y axis offsets for arcs as defined in the NIST interpreter specification.
Exactly how you know which IJ mode to select, I have absolutely no idea but each CAD / CAM has it's own preference and once known then that is the IJ mode to use.
I use 'trial and error' but I am sure others will have a better, foolproof, way to decide between G90.1 and G91.1.
Tweakie.