You could use "osaf.n2.nabble.com", but if you decide to change this domain later to - let's say - "forum.osaf.org", old links to "osaf.n2.nabble.com" won't work anymore. With the default Nabble domain, the node ID is in the domain name and Nabble can redirect old links to new domains. But your change removes this ID and makes this identification impossible.
The screen shot is misunderstood. The ID (number) does remain,
I was thinking of shortening only the 'wordy' part of the name
from osaf-selected-lists.3138168 to osaf.3138168