Multi Domain SSL Certificates or multiple domain ssl certificate secure group of domains or sub-domains or Addon domains for which the certificate is generated. Multi-Domain SSL Certificates include a number of domains purchased, in most cases it is up to 100 domains maximum. When needed add a domain to the multiple domain certificate later and reinstall the ssl certificate to work for last added domain in multi-domain ssl certificate. So do not install multiple ssl certificates i.e. one for each domain or sub-domain. In such a case using Multi-Domain SSL certificates are the correct option to secure domains or sub-domains or Addon domains hosted on one server. Using multi-domain ssl certificates is also a cheap ssl certificate option as it save multi installation costs and also make the renewal management easy.