This allows the use of preauthorized "trusted subjects".
Specifies if the x5c claim (public key of the certificate) should be sent to the STS. Sending the x5c enables application developers to achieve easy certificate rollover in Azure AD: this method will send the public certificate to Azure AD along with the token request, so that Azure AD can use it to validate the subject name based on a trusted issuer policy.
This allows the use of preauthorized "trusted subjects".