[Elecraft] what is the use of IF TX quartz filter on the KREF3 board ?

[Giuliano]

I see on the K3 KREF3 schematic a quartz filter ( 8215 KHz BW= 4 KHz) . 
It is switchable...
someone know what is the use ? In what modes it is switched on? Exist a 
detailed bandwidth  diagram?

Thanks for help
Giuliano I0CG
Italy