The Wildcard SSL certificates are great helpers when you need to protect multi-le sub-domains within the same domain name. The standard single-domain SSL may protect one domain or its sub-domain, for example, domain.tld OR sub.domain.tld
However, the modern infrastructure includes multiple services that use own sub-domains. When you need to protect just a few sub-domains and you already know their names you need to select Multi-domain SSL (SAN) certificates. However, take Wildcard SSL certificates when you have many sub-domains and may not know all sub-domains at the moment.
Wildcard SSL certificate protects an unlimited number of sub-domains, but for the next level of domain only. If order placed for *.domain.tld, that means SSL will protect the domain.tld (base domain) and all its sub-domains (next-level), like any.domain.tld, mail.domain.tld etc, but NOT *.*.domain.tld. Basically, double wildcard certs not exist, same as EV Wildcards.
It is also possible to use Wildcard SSL for mail and exchange servers, as most modern software may work with them like with UCC (Unified Communication SSL certificates).
There is also a new type of Wildcard certs called Multi-Domain Wildcard certificates. They able to protect up to 250 different wildcard domains, that means unlimited sub-domains under 250 domains. Most of our Wildcard certs come with unlimited server licensing too, so you can install different variations of sub-domains on your different servers and IP addresses.
