What does Nz mean in Boss DR-880 documentation?

[ark]

You don't need the manual for this, but if you do, please look at page 153 under the "Bass Tone" headings. There, it lists all the bass "instruments" it can synthesize.

The first few listings look like this:

1 FingerBs 1
2 Finger/Nz 1
3 FingerBs 2
4 Finger/Nz 2

...

18 Slap Bs
19 Slap/Nz

...

23 Upright Bs
24 Upright/Nz

...

Obviously Bs stands for "bass," but what does "/Nz" stand for?

[ark]

One piece of extra information: The "/Nz" instruments seem to sound exactly the same as the regular ones except that when played softly, they're muted, as if one were resting the heel of one's hand on the strings while playing. So I guess that's intended to be the difference between the respective sounds. It's just that I can't figure out why "Nz" would describe that difference.