XL2 and mic attenuation

[David Jones]

Attenuation is the opposite of amplification.
It’s basically a reduction of signal strength, or turning down the input level.

“How and why would you use it on the XL2? ”

Say you are recording a concert where the volume is very loud, and the audio level is overloading.
You could flip the little switch marked Mic Attn. and then manually adjust the input level until a proper signal is reached.