Subject Alternative Name Certificates are public certificates with a list of alternative domains, sub domains and/or IP addresses that can also use the certificate. For example CrucialExams.com, www.CrucialExams.com, api.CrucialExams.com and the IP 126.96.36.199 all in a single cert. Wildcards are a close alternative that supports any sub domain (e.g. *.google.com) but a wildcard could not also be used for gmail.com. To use a single certificate for a sub-domain and entirely different domain a SAN must be used.
Subject Alternative Name (SAN) is an extension to X.509 that allows various values to be associated with a security certificate using a subjectAltName field. These values are called Subject Alternative Names (SANs). Names include:
DNS names (this is usually also provided as the Common Name RDN within the Subject field of the main certificate.)
directory names (alternative Distinguished Names to that given in the Subject)
other names, given as a General Name: a registered object identifier …