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 18.104.22.168 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:Email addresses
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 or Universal Principal Name: a registered object identifier followed by a value.
RFC 2818 (May 2000) specifies Subject Alternative Names as the preferred method of adding DNS names to certificates, deprecating the previous method of putting DNS names in the commonName field. Google Chrome version 58 (March 2017) removed support for checking the commonName field at all, instead only looking at the SANs.