[Dovecot] Difference between using auth_realms and setting up virtual mailboxes in postfix?

Hi, there is a possibility in the future that I might have to support multiple domain names(but using the same IP address). Postfix has something called 'virtual mailboxes' to help handle this.

But I also noticed that dovecot has a setting "auth_realms" where I can specify domains separated by a space.

What is the relationship between the two?