A SAN or Subject Alternate Name is an additional entry on an SSL Certificate in the subjectAltName Field
These additional entries will display in the certificate information (below)

A SAN can be any valid Fully Qualified Domain Name (FQDN); it doesn’t have to share the same root domain in the CSR you enter when placing your order.
Please note: There is no one that can provide an SSL Certificate that will secure .local and .internal or other invalid domains, nor can an SSL Certificate be issued to secure an IP address. This change occurred in 2015.
What is a SAN Certificate?
A SAN Certificate is an SSL Certificate that has entries for multiple domains in the subjectAltName Field and is a true Multi Domain SSL Certificate.
You will find that every FQDN on the certificate is listed in the SANs field when decoding the certificate.

A SAN certificate differs from a Unified Communications Certificate (UCC) in that there is no main domain name (Common Name/CN) listed on the SSL Certificate.
The SAN Certificates that we offer are
PositiveSSL + Multi Domain
Trustico® Multi Domain
Please note:
If you are ordering a SAN Certificate, make sure to enter during the order process every FQDN you require to be on the SSL Certificate. Our ordering system does not import SANs listed on a CSR that you enter.