A multi-domain certificate is designed to secure many domain and/or subdomain 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.
Multi-domain certificates can secure the combination of completely unique domains and subdomains with different TLDs (Top Level Domains) except for the local/internal ones:
Note! Multi-domain certificates do not support www subdomains by default. If you need to secure both www.example.com and example.com with one Multi-Domain certificate, both hostnames should be specified during the certificate activation. Check this article for more details.
Another advantage of multi-domain certificates is that they do not require separate dedicated IP addresses for the hostnames. If the server that hosts your domains does not have SNI technology and thus, does not support the proper functioning of separate certificates on one IP address, the multi-domain certificate will solve this issue. 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 need to renew just one certificate instead of several ones which takes fewer efforts.