The ACS formats the RReq per B.8, establishes the mutually authenticated link with the DS, sets the completion and cancellation indicators on abandonment, sends one RReq for every ARes with status C; the DS validates and forwards it to the 3DS Server at the URL stored from the AReq; the 3DS Server validates, formats and returns the RRes over that link (the merchant may now authorise, or first confirm the cardholder did not abandon); the DS validates, logs and relays the RRes to the ACS, which validates it.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.