From what Decoder Pro shows in their software and what Larry Puckett states in his videos where he discusses the use of Decoder Pro, the address can be either Long; or, Short and it doesn't matter. He suggests that the Address be the locomotive's Road Number, what ever that is. I have several Short Address and several Long Addresses on my Northern Pacific Layout. When it comes to giving an address to a locomotive, Decoder Pro allows you to use two, a Long and a Short Address. In other words, as an example, I have an N.P. GP-7 with Road Number 553, this is my locos Long Address. Larry would also give the loco a Short Address as 53, in this way if he's at his club and someone is already using 553, he can run his loco under the Short Address 53. I don't run at a club; so, I don't use this method. I don't know if you can do this without the use of Decoder Pro.