While I'm no expert, I have done a couple workshops with John Ding (also a disciple of Ip), who described the difference of Crane, Tiger and Snake the same way as Robert Boyd does at his website. Essentially, high, medium, low. It is interesting though, that Robert Boyd continues with this --
"The Snake Style of tai chi requires great flexibility of the spine, hips, rib cartilage and internal muscles of the abdomen and upper thoracic ribs and back. The serpentine movement of the snake style moves the center of gravity using 'core' muscles. These core muscles strongly root the foot at the completion of each posture, sending powerful jin energy through the spine to the hands. The result, over time, is the legendary 'iron in cotton' phenomenon that gives speed, power and sensitivity to the hands and arms. This is the key to the successful use of tai chi in self-defense." --
-- as it seems to me that is what all Tai Chi Chuan should be about and as it is described in the Classics.
I also remember learning that for each frame height (ie- high, middle, low), there are three frames sizes (large, medium, small). If you were to work with all of the combinations possible, there would be 9 variations of form. An example might be Low Posture/Large Frame or Middle Posture/Small Frame. I can't remember where I learned that now, but it always made sense to me.