The Logic Off Topic list

I did not find an explanation in the manual or on the RME website:

The Fireface/Totalmix manual says the setting "Hi Gain" for the  
outputs gives me +15 dB Headroom. There is another statement that "Lo  
Gain" on inputs gives me also +15 dB Headroom. Could someone  
enlighten me what that means? Does it make sense to enable both or  
when should I enable the input "Lo Gain"? I can't see a difference on  
the level meters.

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Peter Ostry