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Here's the SAML document that the CarRental service receives
<Response xmlns="urn:oasis:names:tc:SAML:2.0:protocol"
                   ID="fff011234"
                   Version="2.0"
                   IssueInstant="2005-01-31T12:00:00Z"
                   Destination="https://www.CarRentalInc.com">
    <Status>
        <StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
        <StatusMessage>Success</StatusMessage>
    </Status>
     <Assertion xmlns="urn:oasis:names:tc:SAML:2.0:assertion"
                       Version="2.0"
                       IssueInstant="2005-01-31T12:00:00Z"
                       ID="abe567de6">
         <Issuer>http://www.AirlineInc.com</Issuer>
         <Subject>…</Subject>
         <Conditions NotBefore="2005-01-31T12:00:00Z"
                              NotOnOrAfter="2005-01-31T13:00:00Z"/>
         <AuthnStatement AuthnInstant="2005-01-29T09:32:00Z">
             …
         </AuthnStatement>
     </Assertion>
</Response>
Notice that there isn't an InResponseTo attribute (since it is an unsolicited response).
In the <Subject> element there is information that says, "The bearer of this assertion is the subject of the assertion."  More about this later.