Generally, an SSL certificate is issued to a single domain name only. But in case, you are using a number of domains and subdomains for your business, you can go for Wildcard SSL.
Wildcard certificate is issued with a common name of *.companydomain.com and a Subject Alternative Name (SAN) of companydomain.com. The wildcard certificate can be used on any subdomain of companydomain.com and SAN ensures that SSL works even without a subdomain. For ex, if a wildcard certificate is issued for *.znetlive.com, then it can be used for securing mail.znetlive.com, www.znetlive.com or anything.znetlive.com and also znetlive.com.
It saves cost as you buy only a single SSL certificate for multiple subdomains and with a single certificate, administration also becomes easier. But a limitation of Wildcard SSL is that it only supports first level subdomain and a disadvantage is, that if one subdomain is compromised, then other subdomains are also compromised. So if you need to revoke one subdomain’s certificate, then you will need to revoke it from all the other subdomains.
For any new online business, the choice of SSL may seem like an inundating task. So, knowing about different SSL types will help you to decide the best option for you and your business.