What exactly does "bSpatializationIsExternalSend"?

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