Join Date: Feb 2009
Location: Austin, TX
Bikes: Giant XTC Team Custom XC bike, Nashbar tourer custom commuter
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I don't think it's anything official, it may very well indicate differences between product lines that are more than marketing gimmicks, but I don't know what bikes you're looking at so it's difficult to say.
All mountain is different from XC, characterized by a bit more weight, more durability and stopping power, less emphasis on high speeds (maybe a 2 chainring front crank with bashguard or a 34t cassette), and full suspension frames (though there are certainly all-mountain hardtails and a select few full suspension XC frames).
Marathon and regular could be a weight thing, it could be ergonomics related (better seat, easier shifting levers), but I don't recognize it as a standard.... at least all-mountain is fairly differentiated in terms of usage though it may borrow parts from XC and DH all the same.