The validation process depends on the type of SSL Certificate. The only common feature for the multi-domain certificates we provide is that each domain in such a certificate should be validated before issuance.
The domain-validated certificates will give you an opportunity to select the most suitable verification method for each domain: approval email or alternative HTTP-based validation method. You are not obliged to use the same validation method for all domains inside multi-domain certificate and are free to choose the one that suits you most.
When you are considering the domain with organization or extended validation, it is necessary to remember that all the domains inside the certificate should belong to the same company.
The organization-validated as well as the extended-validated certificates come with a site seal which can display only one company name. The EV certificates provide more information about the company in both certificate details and Trust Logo. The domains belonging to different organizations require separate OV/EV certificates.