trying to understand locals and hosteddomains, is the following correct?

locals is a single file; hosteddomains is a directory whose files are catenated together.

hosteddomains is a superset of locals.

the format of each is the same, typically mydomain.com .mydomain.com the second representing all subdomains of mydomain.com

hosteddomains must include each hostname with DNS AAAA, A or MX to this machine. (locals, if no hosteddomains).

the primary difference is that mail accounts using local domains do not need to specify the domain as part of the mail address. conversely, there cannot be two local domains having the same mail user, eg, there cannot be use...@domain1.com and use...@domain2.com.

hosteddomains is sync'd with makehosteddomains; locals is sync'd with makealiases.

what else?

aliases:

if the domain is local in the mail of an alias, it cannot be specified.

if the domain is local in a maildrop of an alias, it cannot be specified.

you cannot, for example, have use...@local1.com go one place and use...@local2.com go another if local1.com and local2.com are local domains.