Its pretty safe to say that when anyone talks about a "10mm" axle, they actually mean a 3/8. You can get actual 10 mm axles but not many hubs have them. They wont quite fit in some tight dropouts and, obviously, they need different wheelnuts and hardware. You almost wont notice the different, but a 3/8th nut wont fit on a 10mm axle, and the other way round will be lose.
Axle converters work great for fitting 3/8th wheels in a 14mm dropout. (they'll prob work on 10mm too, the washers aint usually aint a very tight fit)