“Threads users are also expected to be able to limit replies to a post, which means that it would not be shared with any federating service, and will only remain within Threads itself.[20]”
straight from Wikipedia. There’s the green bubbles. “Threads Users” and “Unpersons”.
Email is a good example of equitable federation - beyond the email address you can’t generally tell what email software or provider is being used.
“Threads users are also expected to be able to limit replies to a post, which means that it would not be shared with any federating service, and will only remain within Threads itself.[20]”