Addresses now explains its three kinds of address: a mailbox stores mail and has its own Inbox; an alias has no Inbox and delivers to one or more mailboxes, which can send from it; a reject address refuses mail. Add address repeats this, and says that an alias can be added once the domain has a mailbox. Domains says what a domain is and where its addresses are added.
Mail explains mailboxes, aliases and domains
Mail's Addresses and Domains pages, and Add address, now say what a mailbox, an alias and a reject address are.