USB audio "dummy" selector units

[jb]

This datasheet is pre-Win7 and no longer manufactured, so I'm not sure if this is still relevant:

The mixing unit MU7 in the headset firmware is a dummy unit and allows the WIN operating system to parse the descriptor correctly. Mac OS does not recognize the recording path if MU7 is present. For that reason it is possible to “short-cut” MU7 with an EEPROM based switch.

Newer version (2008) has a different topology and says:

The selector unit SU8 in the headset firmware is a dummy unit and allows the Windows operating system to parse the descriptor correctly.

And another:

The switching units SU7 and SU8 in the headset firmware are dummy units and allow the operating system to parse the descriptor correctly.

So it seems this modified version works on both Mac and Windows.  I see similar do-nothing units in products by other companies, too.

Does anyone know what specifically is not recognized, and whether dummy units like this are still required in modern Windows/Mac OS?

Also found a main.lst reference in the SDK:

Dummy selector unit to display microphone at the record slider

[jb]

Found another product with a more clear description:

f) SU (Selector Unit)
Selector Unit is unnecessary for the AK4571's topology. Windows application does not display playback MIC volume slider if the AK4571 does not define Selector Unit.

[Jan Axelson]

I'm sorry, I don't know the answer.

However, I applaud the vendors for explaining the reason for including non-functional units.