configuration files info is missing

Greetings

I'm writting some kind of web based repository controller/monitor. Unfortunally i have problems with .repo files. There is baseurl and mirrorlist, both documented, but one information is missing. If I specify both of them in one file, which one YUM will use to connect to repository? Is this a logical choice (eg. baseurl always before mirrorlist) or a physical choice eg:
[reposection]
name=reponame
baseurl=http://use.me.first
mirrorlist=http://do.not.use.me.or.use.me.if.baseurl.fail

There is one more thing. If both are used, are they merged?
Thanks for attention and have a nice day :>

Chris.