Unified Communications Certificates (UCC) are SSL Certificates that secure multiple domains and multiple hostnames within a domain. They allow you to secure a primary domain, and up to 99 additional Subject Alternative Names, in a single certificate. UC Certificates are ideal for Microsoft® Exchange Server 2007, Exchange Server 2010, and Microsoft Live® Communications Server.
In the Subject Alternative Name field, or SAN for short, any number of different domain names or common names can be entered enabling the certificate to work on any of the included domain names. This allows one certificate to secure both internal network names as well as external domain names.
For example, you could get one UC SSL Certificate to cover all of the following:autodiscover.contoso.com
This can provide significant cost savings in many situations because you need only certificate rather than different certificates for each domain or hostname.