It means they cut most of the chamber ahead of time and it's up to the end user to finish the job. Often done by hand with a reamer.
Short usually means they stop the chambering job before it's to proper length and then the user finishes off the chamber, by hand with a reamer. Looks similar to tapping a hole. It's also the most common way I see them shipped.
Long would mean (I guess) that they ran the reamer in longer than necessary for the end user to simply take some off the "face" of the blank to essentially shorten up the chamber until it's properly head spaced. Would most likely have to be done via lathe.
Either way it's up to the user to check the headspace after with gauges.
You can NOT simply twist on a barrel so done without finishing the work.
I would imagine the short appeals to those without a lathe, the long to those with and don't want to buy reamers for a one time deal.
I can't imagine buying it long/short and taking it to a gunsmith to be finished on his lathe as you may as well have him cut the whole thing, IMO that would probably be the best for accuracy.
