What is a WS-Federation identity provider in Azure ACS and how do you define them?
WS-Federation identity providers are custom identity providers that are configured in Windows Azure AppFabric Access Control Service (ACS) ...

https://www.interviewquestionspdf.com/2017/02/what-is-ws-federation-identity-provider.html
WS-Federation identity providers are custom identity providers that are configured in Windows Azure AppFabric Access Control Service (ACS) using WS-Federation metadata.