The soulname is an expression of the totality of a sentient being. For the High Ones (who had no identifying physical features because they could mould their bodies like clay) it was a way to maintain individuality in the face of almost complete telempathy.
When the High Ones crashed and their descendants were forced to find ways to survive, those tribes that maintained a use of telempathy found it more worthwhile to protect their souls by using one name among the tribe while reserving the soulname for their own personal strength and for their lifemate (through Recognition).
Those tribes which did not need telempathy to that degree ended up using the soulname as a tribename and not making a distinction between the two.
Things get stickier with Blue Mountain and the Go-Backs. The Go-Backs are at least partially wolf-blooded which means that they have about half a soulname (like the Wolfriders for whom soulnames are truncated to a syllable). I would hazard a guess that since they don't tend to use telempathy that they use their soulnames as their real names and those names tend to be short and harsh (like their lives). In Blue Mountain the Gliders tend like their parents and grandparents to use their soulnames as their real names with a few notable exceptions. Winnowill, both Doors, Egg, and Brace (and I'm not going to get into Hatrack et al) all have what appear to be usenames. Egg is actually Aurek. I believe that this is an expression of the fundamental heresy of Blue Mountain. Instead of simply doing a job for untold millenia, like Orolin in the Palace, Aurek *became* his job, *became* Egg. He lost his Name in his Work.
In any case, I hope this explains the situation with those immortals who actually use usenames and not their soulnames like Suntop et al.