I believe that is the inner diameter of the 'hobbled' part of the bolt. I inherited those settings (or definitions) when I got the kit. I modified some of the things like the gear ratio on that line. But I did not create the original version of that definition. I did do a calibration early on in the build process and the extruder did put out the correct amount of filament, so until I changed the gear ratio, I left well enough alone.