can someone explain the difference between dovecot, virtual, local, or relay in postfixadmin when creating new domains?

i figured out relay is for backup servers and local is for system accounts but still working out if we should be using dovecot, virtual or both.

we have three separate domains with three seporate IP's, should they all be virtual?

also, in dns we have mail.example.com, is the mail server for example.com so do we have to add both domains in postfixadmin and if so should they be relay,local, virtual or dovecot?  sometimes i'll see mail for root@mail.example.com since mail.example.com is the host name of the mail server... not sure if that's good or what.

i guess what i'm trying to find out is what's the very minimal and best config when adding domains in postfixadmin from a iredmail install.

sample situation:
user@example.com
smtp: mail.example.com:port
system generates root@mail.example.com

user@example2.com
smtp: mail.example2.com:port
system generates root@mail.example2.com

what needs to be done in postfixadmin to make this work the best?