When Identity Server is configured to federate with multiple external identity providers, you can specify the list of authentication contracts that an external provider can execute. This configuration allows Identity Server (acting as service provider) to automatically select the external identity provider without the user having to click on the external provider's card.
Authentication contracts in Identity Servers can be configured with an authentication class reference. This reference can be used in federating with external identity or service providers that only respond to AuthnContextClassRef in the authentication request and response. For information about setting up the contract mapping and adding contracts to the satisfiable list, see Modifying An Authentication Card for Liberty or SAML 2.0 and Configuring Authentication Contracts.