Hi,

only, what i personally found out. - so no guarantee.
And i can only tell what i got on a mac about this (but should be similar to win):

afaik, the rme.totalmix.default.xml can be used to predefine the params before a unit is plugged in for the first time to a pc/mac. and before TM is started for the first time.

As an example to demonstrate, you could try this (would be interesting for me, too if it its exactly the same on win):
- close TM (really exit it in the taskbar)
- backup the whole directory C:\Users\<loginname>\AppData\Local\TotalMixFX anywhere
- delete all files in this directory
- just create one simple file, named rme.totalmix.default.xml with this content:

    <Default0>
        <Window>
            <val e="WinPos.left" v="0"/>
            <val e="WinPos.top" v="0"/>
            <val e="WinPos.right" v="269"/>
            <val e="WinPos.bottom" v="1042"/>
            <val e="ScaleType" v="1"/>
            <val e="FuncType" v="0"/>
        </Window>
        <Cache0>
            <val e="ShowFX" v="1"/>
            <val e="HideControlStrip" v="1"/>
            <val e="CloseControlRoom" v="1"/>
            <val e="DeviceName" v="Fireface UFX III (YOUR-8-DIGIT-SERIAL-ABBREVATION)"/>
        </Cache0>
    </Default0>

- open TM
- you should see a minimal/odd window with just the FX shown, without Control strip
- scale the window, make any settings (like showing the strip, etc...)
- close TM completly again
- you should notice that now, some more files are created
- including last.FirefaceUFXIII1.xml (btw: this seems differ to macs - on mac its:  last.FirefaceUFXIIIYOUR-8-DIGIT-SERIAL-ABBREVATION.xml
- from now on, TM takes always the settings from last.FirefaceUFXIII1.xml (on win) and ignores rme.totalmix.default.xml. so you can close/open TM again and your last settings are kept...

=> My Final thoughts (again no guarantee - just putting things together):
The rme.totalmix.default.xml seems to be useful, in a scenario like a studio for rent f. ex...
Imagine, the studio offers one/or more unit/s + pc/mac for customers.
After a customer has finished his session, the pc/mac ist set up from a fresh image, which just contains this file in its C:\Users\<loginname>\AppData\Local\TotalMixFX dir. so the next customer has a fresh environment, with default routings (to DAWs/MixingConsoles), settings, windows positions on several monitors, etc...
-> And this file can be also expanded by adding <Default1...>... and so on. for even more than one RME unit, too...

i already tried, too document all possible params in the configs - but i did not find a generic/logic sheme to cover all units and all unique params (and all files/filetypes).

Would be nice to know if you could confirm, that on win the sheme is similar, or if i am wrong with sth...

hope it helps a little.

regards, meg