In this case, given that it’s a request for a filter option, it would be observations of endemic species.
That said, lists would be good too.
Being able to filter observations this was would also help us update iNat info to be more accurate. In my area (limestone island ecosystem in SE Asia), there are a lot of endemic species, but many of them are not marked as such. It would help us fill in gaps in both labeling and observations, and having averified list of observed endemics also helps us push the National Park and provincial politicians we work with for stronger and more effective protective and conservation efforts.