Importing subdomain names from a DB. Then what?

Greetings, Please forgive the newbie question. I am asking to get an understanding of the process, so I am asking the question in the extreme. I understand that I can import a list of thousands of names for subdomains in seconds. Even 20K names, should I choose (though I anticipate 500). Is that correct? Then what? You hit a button after importing your list and they instantly exists? Do they need to propagate? Thanks for the help. - Frank