This certificate will only cover one domain address with the exception of www. where it will cover both www and the root domain.
e.g. www.domain.com will cover both www.domain.com and domain.com.
You can also cover other addresses like secure.domain.com, if you need to cover more we recommend a Wildcard certificate.
This certificate will cover a domain and all of its first-level sub-domains.
e.g. secure.domain.com, domain.com, firstlevel.domain.com and www.domain.com.