I know this is a widespread assumption, and maybe even the descriptions on the web site suggest this kind of behavior. However, based on my looking into these topics, starting from/confirming other people’s findings, I don’t think that is how it actually works.
What is definitely true, though, is that when a client is configured with an unnamed zone, and there are no servers in that zone, so that the client gets handed servers from the enclosing zone as fall-back, that obviously means that those servers in other zones will see this as a request coming from outside their own respective local zone.
Honestly, I’ve always wondered about the “@” zone, and how it works, i.e., under which circumstances a client would be handed a server really from somewhere across the globe. The only thing I could come up with is that should a continent zone also not have any servers in it, the fallback would then be to the global “@” zone. But as there are few continent zones that don’t have any servers in them, this is hard to verify. And the flights to Antarctica were sold out before I could get a ticket to test from there …
Another question on my mind is how the netspeed setting is applied when a fall-back is active. It mostly makes sense within a country zone, and as discussed variously in this forum, netspeed settings cannot be compared across countries, i.e., same netspeed in two countries can result in widely varying actual traffic.
So how does that square when servers from different country zones are to be fed into a single DNS response? Only thing I can think of is that even then, the netspeed is applied as is, comparing against the netspeeds of servers in other zones, but that this is not really that relevant in this case because the load generated that way is perhaps negligible in comparison to the load generated by a local zone. I.e., the netspeed controls primarily the traffic from the local zone, and that it might not be “accurate” to compare servers across zones doesn’t matter because there is too little traffic from that side to actually make a difference. Or it is factored in, i.e., it is anyhow not possible to universally say “netspeed setting X will generate traffic load Y” across all zones, but it is specific to each zone. Thus, operators anyhow need to experiment with the setting and find the “right” one for themselves, and that process implicitly takes into account also traffic coming from other zones.