Same Genus, different Species. Example. Lions and Tigers breed together. Produce fertile litter. Litter breeds with another Lion or Tiger. Produces a Litigon.
That’s not the norm though. There is relatively little genetic variation in the entire feline genus, despite the large variety of appearances. Same goes for the canines. That is why they can interbreed and still produce fertile offspring in hybrids.
The main barrier to producing fertile offspring seems to be the variation in chromosome count across the genus, and almost all feline species have 38 chromosomes. Some South American species lost a couple chromosomes along the way and only have 36. If you try to breed a normal cat or big cat with one of those species, any offspring would be sterile, except perhaps in the rarest of circumstances.
Similarly, canines all have 78 chromosomes, except for a couple of species of jackal, so they can all produce viable, fertile offspring.
This is not the case with most other genus of animals. Usually, the variation in number of chromosomes tends to be larger, which makes hybridization rare, and fertile hybrids even rarer.
The terminology is arbitrary and changes according to whatever definitions scientists are currently using for convenience in their work. Inn recent years I’ve noticed a tendency for scientists and environmental activists to split species up [for example a very common nonendangered species like the Sun Conure] into multiple “new” species in order to claim one or more or even all of the “new” species are thus endangered and each new species and its habitat are in dire need of protection.
In fact, Sun Conures easily breed with similar looking Gold Capped conures and Jenday Conures and even the differently colored Nandays too, and all produce viable young. I think they are more accurately described a natural breeds or races of sun conure than as wholly separate species, and the breaking up of what was formerly known as sun conure into a bunch of new “species” on the basis of wholly superficial if not imaginary differences is just political.
For all practical purposes, these different looking humans are all equally human, just of a different race.