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<AuthnRequest xmlns="urn:oasis:names:tc:SAML:2.0:protocol"
                          ForceAuthn="true"
                          ProviderName="CarRentalInc.com"
                          ID="abe567de6"
                          Version="2.0"
                          IssueInstant="2005-01-31T11:58:00Z"
                          Destination="https://www.AirlineInc.com">
    <Subject xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
        <NameID Format="urn:oasis:names:tc:1.1:nameid-format:emailAddress">
            j.doe@acompany.com
        </NameID>
    </Subject>
</AuthnRequest>
This is the subject for which we want authentication info. The Format attribute indicates how the subject is being identified (in this example the subject is being identified by his email address)