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Microsoft Knowledge Base Article 295663 contains the following summary:
This article describes two methods you can use to import the certificates of third-party certification authorities (CAs) into the Enterprise NTAuth store. This process is required if you are using a third-party CA to issue smart card logon or domain controller certificates. By publishing the CA certificate to the Enterprise NTAuth store, the Administrator indicates that the CA is trusted to issue certificates of these types. Windows CAs automatically publish their CA certificates to this store.
The NTAuth store is an Active Directory directory service object that is located in the Configuration container of the forest. The Lightweight Directory Access Protocol (LDAP) distinguished name is similar to the following:
Certificates that are published to the NTAuth store are written to the cACertificate multiple-valued attribute. There are two supported methods to append a certificate to this attribute.