Supply-Side Platform and Supply-Side Agent are free to communicate in any way they choose, as this communication is not part of Adshares Protocol. Thus, any interactions between Supply-Side Platform and its Supply-Side Agent (e.g. notifications about Impression Events) are not covered by Adshares Protocol.
Whereas the communication between Supply-Side Platform and its Ad Select Module might be part of Adshares Protocol in case Ad Select Module is externalized and delivered by a third-party Software Provider. If this is the case, Supply-Side Platform calls its Ad Select Module to outsource to it the process of generating a list of Creatives.