XM Radio calls it a radio ID number but it's actually an antenna ID number.
Jim (hd-dude) installed mine with the antenna up in the fairing, permanently mounted and hidden out of the way. Very clean job. So I want to use the XM portion in my trucks and cars so I buy another antenna, thinking "well, the radio ID is already subscribed to and registere" so shouldn't be any issue.
I put it in my truck with the new antenna and the GPS works fine but I can't get any reception. I call 1-800 XM (or whatever the number is) and they ask me for the radio number. I gave them the number that I had wrote down, from when it was new and they beemed the signal again and said "it should be working". I tell them it's not. Anyway, to make a long story short.......I finally figure out that they need the number off of the back of the antenna and want to charge me a new subscription fee for it, as well. They sent me a signal for just a few minutes, so that I could be sure that was the only problem but I've already got three XM radios going......decided I didn't need a fourth.
So, I've got a new Garmin 2730 XM radio antenna should somebody find theirself needing one. By the way, being the electronics wizard that he is, Jim did shorten the cable that goes from my GPS to he front of my head unit, successfully. So that looks pretty clean, too.