128
SubjectQuery
<SubjectQuery xmlns="urn:oasis:names:tc:SAML:2.0:protocol"
                         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>
</SubjectQuery>
"Hey IdP, give me the assertions that you have which match this subject."
see example08