oversubtraction corner frequency

[HyeeWang]

In the matlab script specsubm.m ,which can be find in voicebox,
"oversubtraction corner frequency " is used

to compute the oversubtraction factor. The code of specsubm.m is a
program of spectral subtraction method based on minimum statistics.

% 6: fo = oversubtraction corner frequency (800): high=distortion,
low=musical

osf=po(11)*(1+(0:nw/2).'*fs/(nw*po(6))).^(-1);
...
os=zo*os+(1-zo)*(1+osf.*pn./(pn+pxn));

But in the listed literature of spectrum.m, I did not find any
information about "oversubtraction corner frequency".
Where can I find the concept about "oversubtraction corner
frequency"? How to apply it?

Regards
HyeeWang