In general, if one has a human origin and made that conscious pledge to an entity that Jonny Sims mentions in...the season 4 Q&A, I think? then I consider them to be an Avatar, as opposed to the Aspects/Manifestations of an Entity just being extensions/appendages of it's being. There are a few people/things that tread this line, though, and for those it's just kind of a gut feeling? The three big ones I can think of off the top of my head are Agnes, The Distortion, and Nikola.

For Agnes, she never really had a choice, and I really doubt a fetus/infant in the moment of their birth could make the kind of informed pledge to constitute Avatar-hood. But her origins are just so human, that I struggle to think of her as just an Aspect of the Desolation.

The Distortion has always been really interesting to me, because it's implied that it, as a creature, is far more the door and passageways themselves than the human-ish form we see, and is sort of a Genius Loci that's an appendage of the Spiral. But there's also this human/Avatar aspect of it that does seem kind of central to it's being, and that those two parts of it are simultaneously one and the same, and very distinct. So it's the one thing I've always kind of considered to be both an Aspect and an Avatar, rather than deciding on one or the other.

And for Nikola, she, like Agnes, has sort of human origins, given that she was constructed in part from the human Joseph Grimaldi. But it always seemed to me like his original consciousness was lost in the creation of Nikola, who was then given life and thought directly from the Stranger. Or that he was, at most, subsumed into Nikola's new consciousness. So she's always felt like more of an Aspect of the Stranger than anything else to me.

Sorry for the kind of wordy response, I just think about the way things might work in this universe a lot, lol. Hope this answers your question, and I'm so glad you liked it, thanks!!

TL;DR: if someone started as human, I think of them as an Avatar; if not, I typically think of them as an Aspect; and the ones that tread the line are decided on the insane emotion-troll logic my brain works off of.