I am looking for the documented observability boundary of Audio Unit version 3 MIDI output in Logic Pro. When an Audio Unit submits events through AUMIDIOutputEventBlock, is there a supported public interface or documented ordering guarantee that can establish that all events accepted for a particular output route and stopped session have been processed, with none still pending for later delivery? I mean that component's route, not unrelated tracks or every queue in the host. I have reviewed the public documentation for AUMIDIOutputEventBlock, scheduleMIDIEventBlock/AURenderEvent, render observers, transportStateBlock, contextPresentationLatency, tailTime, and AUMessageChannel. I have not found a route-completion acknowledgment. A successful submission callback, stopped transport, received event list, or finite silent recording seems to describe a different observation point from completion of all pending delivery. Does Logic support a documented AUMessageChannel protocol or other public mechanism for this purpose? If so, which Logic/macOS versions support it, what is the completion notification's scope and clock domain, and what are its ordering and caller-thread requirements? In particular, does it remain available if transport stops and no final render callback occurs? If receiving every expected event at a downstream Audio Unit can establish completion instead, what documented guarantee rules out later delivery or duplicates from the same stopped session? Instrument processing, effects tails, and actual hardware playback are separate observation boundaries; I am not treating their completion as implied by MIDI receipt. Confirmation that this internal state is not exposed by a supported public interface would also be useful. This is an API-capability question, not a report of an observed Logic defect. I am not seeking private APIs, confidential implementation details, queue flushing, process instrumentation, virtual drivers, or audio-device reconfiguration. No project source, logs, recordings, or account information are included.