What is Master Device in MIDI?
MIDI system needs to know who is the master. 

It is for the master volume adjustment when you have multiple output devices.

Each output device can have its own volume adjustment preset, but the master volume will control overall Volume.

The MIDI has 2 options to set up:



The Create Multi-Output is self explanatory.

The Aggregate is:


  You can combine several audio devices to use as a single device, known
  as an “aggregate device.” By aggregating devices, you can increase the
  number of discrete audio inputs and outputs without purchasing more
  expensive multichanneled audio equipment. For example, if you have an
  eight-channel audio device and a two-channel audio device, you can
  combine them to work as a single ten-channel audio device.
  
  For the aggregate device to work correctly, the sample rates of the
  combined devices in the aggregate device must be set to the same
  sample rate.