Categorizing the ABFAB entities shown in Figure 4 according to the taxonomy of terms from [RFC6973] is somewhat complicated, as the roles of each entity will change during the various phases of ABFAB communications. The three main phases of relevance are the client-to-RP communication phase, the client-to-IdP (via the Federation Substrate) communication phase, and the IdP-to-RP (via the Federation Substrate) communication phase.