TElExtendedKeyUsageExtension is a descendant of TElCustomExtension class.
The following paragraph is taken from RFC 2459 (Housley, et. al.), part 188.8.131.52:
«This field indicates one or more purposes for which the certified public key may be used, in addition to or in place of the basic purposes indicated in the key usage extension field.
This extension may, at the option of the certificate issuer, be either critical or non-critical.
If the extension is flagged critical, then the certificate MUST be used only for one of the purposes indicated.
If the extension is flagged non-critical, then it indicates the intended purpose or purposes of the key, and may be used in finding the correct key/certificate of an entity that has multiple keys/certificates. It is an advisory field and does not imply that usage of the key is restricted by the certification authority to the purpose indicated. Certificate using applications may nevertheless require that a particular purpose be indicated in order for the certificate to be acceptable to that application.
If a certificate contains both a critical key usage field and a critical extended key usage field, then both fields MUST be processed independently and the certificate MUST only be used for a purpose consistent with both fields. If there is no purpose consistent with both fields, then the certificate MUST NOT be used for any purpose.»