[Elecraft] K3 Noise reduction question

[Chris Meagher]

Hello,
When I hold NR button to adjust noise reduction, I get levels F1, 1 to 4, then F2 1 to 4, up to F4.
Clearly, 1 is least, 4 is most, but what do the F's represent? (Can't find any reference to this on page 24 of the manual.)
I wondered if F refers to filter, if so then why not 5?

Thanks for any answers.

By the way, installed latest beta firmware, and so far so good.

Chris  VK2LCD
K2 3666
K3 685