During the initial communication phase, the client sends an NAI (see [RFC7542]) to the RP. Many EAP methods (but not all) allow the client to disclose an NAI to the RP in a form that includes only a realm component during this communication phase. This is the minimum amount of identity information necessary for ABFAB to work -- it indicates an IdP that the principal has a relationship with. EAP methods that do not allow this will necessarily also reveal an identifier for the principal in the IdP realm (e.g., a username).