Why we choose (256)in spectrum?

I use code Matlap for hiding audio in audio ( I mean stagenography) And I faced some problem with one line

[y,fs,nbits]=wavread('snd1.wav');
[y2,fs,nbits]=wavread('snd2.wav');
Yleft=Y(:,1);
Y2left=y2(:,1);
Figure;
Spectrum(yleft,256,fs);
Figure;
Spectrum(y2left,256,fs);

The question here is Why we choose (256)in spectrum? And can I change it or not ?

[Walter Roberson]

Please show us

which Spectrum

I find a spectrum (lower-case 's') routine in the System Identification Toolbox, but it does not take a single numeric value as a second parameter. I need to know which routine you are calling in order to figure out what the parameter means.