I don’t really have a firm answer for you, but I can talk about my personal experience.
In my personal experience, I have had headmates that were not soulbonds, and soulbonds at the same time. Soulbonds tend not to front as often, and when they do are almost always ‘cofronting’. Likewise, they are much less concerned with what is going on in the outside world and more concerned with internal things/their soulbonder.
For me, when I communicate with headmates, versus when I communicate with soulbonds the experience is qualitatively different. It seems to come from a different ‘spot’ in my head, if that makes any sense.
I have also had soulbonds who later became regular headmates themselves, and I found that their presence had moved and priorities had changed.