Administrators can create a list of trusted certificates by designating all certificates in a particular certificate store as trusted and/or by explictly specifying a list of individual certificates. A trust list can contain either instance certificates or certification authority certificates. If the list contains instance certificates the application will trust peers that use the instance certificate (provided the ApplicationUri and HostName match the certificate). If the list contains certification authority certificates then the application will trust peers that have certificates issued by one of the authorities. Any certificate could be revoked by the issuer (CAs may issue certificates for other CAs). The RevocationMode specifies whether this check should be done each time a certificate in the list are used.

Reference

CertificateTrustList Members
Opc.Ua Namespace