how uncertainty is defined for the mu instruction?

Hi,

I have a question regarding presenting uncertainties in the robust control toolbox function mu , which calculates the boundaries of mu to the given system :

       rsexamp;
       omega = logspace(-2,2,200); 
       M_g = frsp(M,omega);
       M11_g = sel(M_g,1:2,1:2);
       deltaset = [-1 0;-1 0];
       [mubnds,dvec,sens,pvec,gvec] = mu(M,deltaset);

here deltaset is only the structure of the perturbations, and in the definition of M there is no uncertainties, that's it, M is a fixed system matrix. how uncertainty defined here? where are exactly the uncertain elements of the system? I tried to follow the documentation provided by the toolbox but it is not clear so far!

regards Basel