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If the IdP is configured for one or more of the methods requested by the SP, then that method may be used. If the Service Provider does not specify a particular method, and the user does not have an existing session, then the default method identified on the relying party's configuration, is is used. If no default is identified there, the IdP will choose one of the available methods; the way this choice is made is unspecified and you should not rely on it being the same from release to release.
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