In Jay’s link I’d like to highlight this response from Minus_Kelvin (who I assume is Aaron McLeran) to clarify the future role of the EndpointSubmix:
“In 4.25 we have the concept of an “endpoint submix” which is where we are going to implement this, hopefully in 4.26, as a fundamental feature of submixes”
So if I understood it correctly, the EndpointSubmix currently is not more than a placeholder where it will be possible around the 4.26 release to assign dedicated output channels and different audio devices.