User certificates can be used by end users, smart cards, or applications, such as Web servers. Server certificates are a type of user certificate. For example, if a certificate authority issues a certificate for a Web server, placing its distinguished name in the Subject field, then the Web server is the certificate owner, thus the "user" for this user certificate. Typically, user certificates do not validate other user certificates, except when they are used as a trusted certificate in a user-centric trust model.