Just means they will only be made at the beggining of production, hence they will cease to be made after a certain time, leaving only the regular editions to be made. Kind of like, in comics.
But in comics, I wonder of their motives in slapping Ltd. Ed. on the cover. They think it makes the buyer want it for being more likely to be collected, or whatever. But I doubt that's the same with Symphony X.
Symphony X knows we'd buy it regardless of wether or not there was a "Limited Edition" label on it.