What follows is a skinnied-down, photographer-to-photographer, explanation of my reverse engineering of the differences in the IR modes (unofficial and unsanctioned
)...
Close: Each photo is metered and the appropriate exposure is calculated with a fairly aggressive tone curve applied by the processing engine to give a pleasant but heavy contrast to the photo. The blacks are never blocked-up so they can easily be recovered in post if necessary to increase the effective flash range, but a little too often on close subjects the whites are blown by the heavy applied contrast and are of course unrecoverable. If you don't mind easing out the shadows for increased range and a little motion blur or blown highlights don't bother you then this is a good mode for close to medium range.
Far: Each photo is metered and the appropriate exposure is given a -1 1/3 stop compensation (this is precisely bass-ackwards to what one would intuitively think for a 'Far' flash setting). After capture (I believe) the underexposed image is pushed by a +1 1/3 stops to compensate and very little, if any, contrast is applied. The net effect of these exposure and processing engine gymnastics is to allow for a faster shutter speed to help control motion blur and to help avoid blowing of the highlights from excessive applied contrast. The image produced will be fairly flat in tone but can easily be polished-up in post with a controlled contrast adjustment that doesn't blow the highlights. This is my favorite mode for anything from 4-45 feet because I don't like blown highlights or motion blur and I don't mind polishing-up the contrast on a deserving photo in post.
Field: This is more of a specialty mode to force the camera into exposure parameters that maximize apparent flash throw. The metering system is largely suppressed to help keep it from messing up your long range intentions and the camera nearly always shoots at 1/20th second at ISO 6400. The grain will be noticeable and any objects in the near foreground will consequently be blown to oblivion. Only a large object within 5 or 6 feet of the camera will be enough to cause the metering system to tone down the shooting parameters. This is a last resort mode IMO best used for extremely long range.
I've left off any discussion of the metering system and how the camera interacts with it to keep this short and to the point of your question. If you want more, I'll be happy to give you my takes.
Hope this helps!