If you intend to secure your root domain (“www.domain.com” and/or “domain.com”) with your GeoTrust Multi-domain Certificate, you can get the best value with by using “www.yourdomain.com” as your Common Name in the CSR. This will automatically include “domain.com” within your certificate.
Simply generate the CSR with the Common Name (Domain Name) as www.domian.com and GeoTrust will automatically include “domain.com” (the non-www form) in the certificate when you receive it. Covering both versions has no downsides and enables more flexibility.
If you want to cover a different sub-domain, such as “store.yourdomain.com”, then this does not apply to your order for this domain, because there is no such thing as “www.store.yourdomain.com”. In this case just generate your CSR with the exact sub-domain desired without “www”.