I actually think this is a great question to ask, and I am hoping some (more?) birth professionals come in and answer it.
When I think about this distinction, personally, I think of "low risk" as = someone for whom outcomes would be as good as or better at home vs. the hospital. Probably because I am planning HB and have not yet been PG.
So right now, I assume I will be "low-risk," though I figure that could change when I'm PG. Why do I consider myself "low-risk" or "likely to be low-risk" when PG? (Again, just my very non-professional, intuitive, not at all set in stone answer to this question.)
-I am in good physical health with no pre-existing physical disabilities or diseases to speak of.
-I am <35 years old and should still be when I have my first, at least (not that I'd consider anyone 35+ to be high risk or not low risk, but <35 is pretty clearly neutral or "positive")
I also assume I am "likely to be" "low-risk" because there have been very few in my family who could be considered at all high-risk (IMO). I know that's not everything, but it's the only thing I have at this point, where I have never even been pregnant.
-I had one bio aunt develop pre-e during one of her three pregnancies, though she also has/had a lot of other medical problems.
-One grandmother was suspected in retrospect of having had GD, but there's really no solid evidence of that and her births and babies were all fine (though babies were big).
-Essentially no C/S, no VBACs and no medical emergencies for any of my 1st or 2nd degree female relatives during PG or birth, although one grandmother did have one SB in her 40s (after 5 normal births).
-No multiples. No preemies. No babies in the NICU or equivalent. No non-vertex babies.
Kind of amazing, really, considering I have the 2 grandmothers, mother, 3 bio aunts and 2 bio female cousins with kids. Including the SB, that's out of 17 births (to bio female relatives) in my family. Basically one SB (in the 1960s) and one case of pre-e and that's about it, not even C/S. Pretty good luck/genes/something.
I figure I will be considered "maybe-not-so-low-risk" or "high risk" if I end up with GD, pre-e, any placental issues, multiples, breech or a rarer complication specific to pregnancy. I don't think of GBS+ as taking you out of the "low-risk" category, but maybe that's just me.
I guess that's all off the top of my head.