The question If no one is paid, then what is the difference between a teammate and a volunteer? is closed for needing details or clarity. From the comments I have a feeling that there is a mismatch between my understanding and the commenters' understanding about what volunteer is. Is that correct? What details or clarity does the question need?
For your convenience, I'll quote the two comments that I think may open the door to the solution:
DJClayworth : "There are many projects that don't call their members as volunteers, although they are not paid." That's just a choice of name, in the same way you might call your customer service people "satisfaction engineers". It means something only in the message it sends to the people concerned.
Me: I understand that it's just a choice of name. But I think they don't use it as an euphemism, but truly view teammates/partners as being different to volunteers. I think this is just a the same as startups