Most certificates are issued only for the 1st domain. Exceptions are certificates that have Wildcard and SAN marks in their names. They are designed to protect multiple domains, and therefore their cost is higher. Domains and all subdomains of the same level are protected by certificates of the Wildcard category: certificates ordered for domain.ru will also protect subdomains mail.domain.ru, service.domain.ru, etc. SAN category certificates can protect up to hundreds of different domains on the 1st or multiple servers.