I am putting a front derailleur on a Surly Long Haul Trucker frame. I know from the LHT specs that the front derailleur clamp diameter should be 28.6mm. However, for the Shimano Deore front derailleur that I want, the online shop gives me the choice of "28.6mm/31.8mm/34.9mm" and "28.6mm/34.9mm". What is the meaning of these multiple measurements, and does it matter which of the two I choose if both include "28.6mm"?
Those are the seat tube diameters that the derailleur will fit onto. Bicycle seat tubes vary in size for a range of reasons (which is also why we have different size seatposts).
In this case it looks as though one derailleur is giving you a list of three sizes that work, the other is giving a range. I suspect they're equivalent, since the maximum and minimum are the same - I'd be surprised if either of them won't fit any tube between those sizes.
What matters, obviously, if that you buy one that will fit the seat tube of your bike. So either of the ones you've found should work.
In an effort to not have 16 models of front derailleurs for the same range of price-point bikes, most manufacturers have gone to a larger clamps and shims for the most common seat tube sizes.