I would answer this question very differently for different types of organisms, and I would hope iNaturalist handles it differently.
Birds are quite mobile, and vagrant birds far from their native range are a relatively frequent occurrence. Ornithologists distinguish different types of range: breeding range, wintering range, migration range, and then various other terms like “casual”, “accidental” or “vagrant”. Casual usually means more frequent than “vagrant” or “accidental”.
The terminology of these ranges are useful to experienced birders. If you count 10 nesting pairs of something in an area, you know you don’t need to check against the “accidental” species. But if a strange-looking gull shows up on the beach in winter, or an unusual warbler in migration, it might be time to check those things. I once saw and photographed a Tropical Kingbird in Philadelphia, and a friend of mine saw and photographed a Townsend’s warbler.
With plants, things are very different. Plants don’t fly around and don’t migrate, and you don’t have the same distinction of non-breeding individuals just hanging out vs. breeding pairs, but some of them do have seeds that blow very far in the wind, float in the ocean, or hitch rides with humans traveling far and wide.
I like how BONAP and the USDA Plants database in theory handle these things. In practice though I find their data is pretty incomplete. For example, the USDA database has categories including “Garden Persistent” and “Waif” for things not-quite-fully-established in the wild in sustaining populations. BONAP uses the “Waif” category but in practice has very few records where it identifies species as having this status in a particular county or region. Also, BONAP, which has more thorough and accurate data than the USDA in many cases, still lacks full-fledged and accurate data on relative rarity of various species in various locations. They usually seem to record this data only by state level, which leads to some weird inconsistencies in their maps, which tend to break down whether the species is present or not by the level of individual county.
I don’t know of any site or reliable source that has range map data that makes these distinctions. So…although when it comes to, say, birds, you can get good sources on the possible range of vagrancy for various species, with plants, you can’t really get this.
When you’re talking about the “range” of a species for the purpose of ID, I think the relevant question is…is there a decent chance of this occurring in the wild?
I.e. if something hasn’t been reported in the area but maybe has been reported in the wild 50 miles away, maybe…just to be safe, it’s worth checking the ID against it. If it has been reported in the immediate area, whether or not it’s native is irrelevant…it’s important to check against it for ID purposes. Then people can separately quibble or discuss about its native vs. introduced status.
I don’t know if this makes sense? Like this discussion arose because of the computer ID suggesting species not found in an area…and I think this is really bad. To this end I think we need to use the standard of whether or not a species is reasonably likely to be found in the general area.