On our shared (non-reseller) web hosting plans addon domains allow multiple domain hosting on your account. These can be purchased seperately as an account addon which allows you to host an extra domain for each addon domain you have.
An addon domain is intended to be a completely different site to your root/master domain. If you simply want a mirrored copy of the master domain that is accessible through a different address (e.g. for a different domain extension) you can use a alias domain.
