Ok. Evidently you do not want to believe the manual you yourself posted.
Would you settle for Focusrite's own Support Page? (google works!)
Invalid Link Removed
Applies to: All products
Microphones must be connected to an XLR input using an XLR cable rather than a 1/4" jack input.
The XLR input passes the signal through the microphone preamp part of the interface, the 1/4" jack input does not.
"...passes the signal through the
microphone preamp part of the interface..."
Why did the Focusrite Help Centre need the word "microphone?"
And - again - from the manual:
"... automatically configured for a line
or instrument ..."
Why not just "line?"
Because there are
three options: microphone, line, or instrument. The "line" option is the only one
not using a preamp.
If the only options were XLR=microphone and 1/4"=LINE, then the "LINE/INST" switch would not be needed; the circuitry would simply discover which type of plug is being used and behave accordingly. That switch is not there to designate whether the plug is TS or TRS - the circuitry can do that. The switch is there to take the instrument preamp in or out of the circuit.
A big selling point of this and similar interfaces is that they allow you to plug your guitar right into them. As I stated before, a LINE input does not provide sufficient gain for an electric guitar. And line sources will overload the inputs on guitar amps, because guitar amps have PREAMPLIFIERs built in.
From
Line level, instrument level, mic level explained:
"+4 dBu is "professional" line level, common in modern pro recording gear, and it is about 1.25 V.
0 dBv is an average line level, typical output from rackmount guitar/bass preamps.
-10 dBv is "consumer" line level, common with older and cheaper recording gear.
-20 dBu is roughly
in the neighborhood of a typical instrument's output.
-30 dBu is again
in the neighborhood of a typical microphone or DI box's output."
The only reason I'm engaging in this argument, which I've tried to keep courteous, is that this is an important thing to understand in this context.