From Breaking Worlds

The people of Arginet - and of every other nation in the world - are human.

Not every person remains fully human, however. A strong alignment to one of the elements will eventually transform a human into a fay whose body and mind are suffused with the nature of that element. This can happen at any time, but rarely happens before adolescence; it is particularly common in the Noble Houses due to their strong traditions and resonances from their estates.
In some cases, the term ‘human’ is taken to include fay, but sometimes it doesn’t. Where the distinction is important the term ‘untouched’ always refers to humans without the fay touch.

Since the Breaking, some ancestors have returned to their bodies as unliving revenants. They are instantly recognisable as walking corpses, but they are in most regards as human as any other citizen apart from the fact that they are dead. Arginet is still adjusting to the existence of revenants and their place in the law and in society is unclear.

Other worlds evidently have humans as well, since every offworlder recognisable as a person has been clearly human. (Some of the monstrous creatures that have appeared through the rifts have shown signs of intelligence, but they are sufficiently alien - and usually dangerous - that nobody has yet established if they are on par with humans.) Interestingly no offworlders have been found to be fay.
Different people have drawn different conclusions from this: some take it as a sign that humanity is universal across the multiverse, while others believe it is evidence of a previous Breaking in which humanity spread to other worlds. This is a matter of particular interest to the Seekers of the Utmost.