This test will list CNAME records for a domain in priority order. The CNAME lookup is done directly against the domain's authoritative name server, so changes to CNAME Records should show up instantly. Also called a canonical name records, they act as aliases, pointing to another DNS name.
For example, you might have a CNAME record docs.example.com, which points to ghs.google.com. When you type docs.example.com into a browser, it will bring up your Google Apps account.