OID value: 2.16.840.1.113730.1.12
The value is an IA5String. It is a "shell expression" that can be used to match the hostname of the SSL server that is using this certificate. It is recommended that if the server's hostname does not match this pattern the user be notified and given the option to terminate the SSL connection. If this extension is not present then the CommonName in the certificate subject's distinguished name is used for the same purpose.
URL for further info: http://www.netscape.com/eng/security/cert-exts.html
See also the OID Repository website reference for 2.16.840.1.113730.1.12