I remember this topic has already been discussed somewhere in the depths of this forum...
A soul name is a word-sound-concept used by the telepathic Wolfriders to guard their inner selves against telepathic intrusion (not necessarily hostile).
Something like a "true name", describing all that the particular individual is.
Only a largely telepathic tribe would have the need of soulnames, so Go-Backs and Sunfolk go definitely without them.
Kahvi and Yun might be the only exceptions.