Audio Auxs

The internal mix is 48 kHz stereo. Master and Headphone are always there and cannot be removed.
AUX buses A–H, up to eight. Same job as a console AUX, a mix-minus, or an ISO send.

Mixing Unit send

A Mixing Unit chooses which bus it feeds.
Follow tracks Preview/Program and the T-bar mix. Independent ignores the video switch and mixes Inputs assigned to that bus as they are.

Device assignment

Hardware assignment is Audio Auxiliary in Settings.
Enabled keeps the bus mixing internally with no output device.
Headphone copies Master makes the Headphone bus a duplicate of Master.

Device detail is in Audio, ASIO, and related.