What exactly does "bSpatializationIsExternalSend"?

How do bSpatializationIsExternalSend, bReverbIsExternalSend and bOcclusionIsExternalSend from the FAudioDevice actually work? When and where is the audio sent to?

These are part of the Spatialization Plugin API which allows 3rd party plugins to override and implement their own Spatialization, Reverb, and Occlusion processors, including sends without returns.