Multi-Domain SSL (also known as SAN / UCC certificate) makes it possible to secure multiple FQDNs in a single certificate.
With TLS-SANs (Subject Alternate Names), Multi-Domain certificates provides a convenient option for organizations that want to secure multiple domains without having to manage multiple certificates. Unlike Wildcard certificates, Multi-Domain certificates can secure any list of external FQDNs. Furthermore, you can also include wildcard domains in a Multi-Domain certificate.
Unified Communications Certificates (UCC) are TLS / SSL certificates specifically designed for Microsoft Exchange and/or Microsoft Office Communication Server (OCS) environments. These Multi-Domain certificates (also known as Exchange certificates) uses the Subject Alternate Name (SAN) field to secure the full set of addresses required to operate Microsoft e-mail servers.