"example" = domain name
".com" = generic domain extension
When you say "example.com is a subdomain of com, and com is a subdomain of " this statement is incorrect.
Sub-domains also called DNS records are what goes before any domain name.
"www" is a subdomain, so is "mail" so is "webmail"
@ is domain root, it is used when you submit just the domain "example.com" then you will be using the @.
As stated before by other users, you CAN create a CNAME, but it is NOT recommended be because it will cause conflict with your DNS responese.
"Not possible - this would conflict with the SOA- and NS-records at the domain root" I AGREE.
A domain's manager.