A multi-domain certificate is designed to secure the main (primary) domain of the certificate and several SAN (Subject Alternative Name) domain names.
Multi-domain certificates are mostly used to secure the services of Microsoft Exchange Server (versions 2003, 2007, 2010, 2013), Office Communications Server 2007 or Mobile Device Manager.
The Multi-domain certificate can secure the combination of completely unique domains and subdomains with different TLDs except for the local/internal ones:
Another advantage of multi-domain certificates is that they do not require separate dedicated IP addresses for the host names. If the server that hosts your domains does not have SNI technology and thus, does not support functioning of separate certificates on one IP address, the multi-domain certificate will solve this situation. Being installed on one IP address, it returns the same certificate for all the domain names included in it, making all the domains secured automatically.
In addition, it is easier to keep an eye on just one certificate and manage it when necessary, e.g. reissue or renew it. When the expiration date of the certificate is coming, you would need to renew just one file instead of several ones which takes fewer efforts.