In simpler terms:
SP initiated: User accesses the application. The application constructs the request (in most cases by using MSAL) and redirects the user to B2C.
IDP initiated : User directly goes to the IDP authentication endpoint and then access desired federated application by providing required parameters, such as client_id, state, scope etc. In case of B2C, you can do the IDP initiated authentication by using the "Run User Flow" link.
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