4 part domain names

[Jonathon McKitrick]

: Every unique combination of subdomain.domain.tld could point to an
: arbitray other URL or IP.
: For example
: us.510.mail.example.com = example.com
: de.510.mail.example.com = europe.mail.example.com

I guess my question is this...

if 'us' is the name of the node (machine) and 'example.com' is the
registered domain name, what do the '510' and 'mail' parts uniquely
identify?  Why not just 'us.example.com'?

jm
--
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