An X509 certificate is created following a specific structure. At the beginning of the certificate data is the version number, followed by the serial number. Other common information includes the issuer, the dates the certificate is valid for, and the public or private key data. In the event the certificate is being used with a digital signature it will also be embedded within the structure in proper location. There is even optional information locations that companies may decide to include.
Use Extensions with X509 Certificates to Be Able to Tell Them Apart
Organizations might decide to modify and customize existing X509 certificates to create their own unique versions using this standard. Anytime modifications are made the certificate should be saved in a format to make it easier to distinguish the customized certificates from the regular certificates. The easiest way to accomplish this is by adding an extension to the certificate when it is created. You may also decide to use extensions for specific kinds of certificates, such as certificates containing digital signatures.