The shortest model(s) (6-9 inches) are the BRM versions (Benchrest I'd guess). They have notched legs and as such won't slip when you're shooting.
The model I use most is the LM (9 - 13 inches), it's six (6) position spring loaded notched legs also won't slip when shooting.
There is a model 25C I believe that has three (3) piece adjustable legs. This model allows the shooter to comfortably sit while shooting. It has the friction type infinite adjustable legs.
The "S" series of these same bipods as the Swivel models. I don't have any swivel models but many folks say they're very nice.
Many of the models spring loading is inward loaded (retract automatically), I prefer the outward loaded (spring out) legs (the LM amd "S" LM).
The friction type (infinite) leg adjustment is nice but IT WILL slip on you, it's not a matter of IF it's a matter of WHEN. The notched legs offer a little less adjustment precision but they won't slip while shooting.
I pean the bolt on the legs when I get a new bipod... the nuts will become loose and you may have a non-functioning bipod when least expected. Just a little calibration style "whack" to mash the bolt end nicely to prevent the nut from escaping.
"Model BR: (Bench Rest) Lowest of the Bipods. Most useful for shooting off the bench. Adjustable height from 6" to 9".
Model L: (Light) The world's largest selling bipod, compact and light. Adjustable height from 9" to 13" Weight 11 oz.
Model H: (High) For prone or sitting shooting, it's the best for varmint shooting. Adjustable from 13 1/2" to 23"
Model 25C: The tallest of the Bipods, Three piece leg construction adjustable from 13 1/2" to 27"
Series "M": A new version of the original Leg Notch Bipod. The legs eject by spring action. Height settings in 1" increments.
Series "S": Rotates to either side for instant leveling on uneven ground. Hinged base has tension adjustment and buffer springs to eliminate tremor or looseness. They are otherwise similar to non-rotating Series. Available for ALL models except the "H" model."
Hope this helps.
There is one model that I have found to be of little practical value.. it's the two (2) piece leg Tall model (I forget the exact model number)... It's a little too tall or not tall enough it seems.
Here's the models described via a vendor...