Yes, it is arbitrary. I have chosen usernames 01-abcd567, 02-abcd567, and 03-abcd567 for my 3 stations, and can see them listed and synchronized at following address.
http://adsbexchange.com/sync-2
Initially this page appears with some text starting with “This table shows…”. One has to wait for several minutes, then it loads the page showing my above stations.
You can divert the incoming feed to VRS on your Windows Desktop, and can see it. The detailed method is given here:
Bake a Pi >> Additional Feeders** >> scroll down to Adsbexchange.
Scroll down to last few paragraphs starting with " (8) Segregation of MLAT Feedbacks of Flightaware and Adsbexchange".