[Elecraft] Main Menu--ATTEN function

[CenturyLink Customer]

I recently was setting up my K-3 for interface with WSJT-X software. In checking to be sure that the sound card was connected to Line-In, I found this menu setting. It appears to be a new item from a firmware upgrade, as it is not shown in my users manual. It is positioned between ALARM and LCD ADJ. It is set at 10 db and is not adjustable from the front panel. When trying to adjust this, I get a message of N/A. I assume that means "not adjustable". 

What is this menu item for? Why is it set at 10db attenuation? What functions does it affect? Thanks for any information that can be provided. 

73, Mike Smith 
K0CCM