[fit,errdb] = rationalfit(s,i,j) uses the rationalfit function to construct a fit, an rfmodel.rational object fitting only the (i,j)th element of the S-parameter object s. This syntax is equivalent to rationalfit(s.Frequencies,rfparam(s,i,j),...).

[fit,errdb] = rationalfit(s) uses the rationalfit function to construct a fit, an N-by-N of rfmodel.rational objects (sharing identical poles) fitting all the N-by-N elements of the S-parameter object s. You can directly pass this N-by-N fit to freqresp, ispassive, passivity functions. This syntax is equivalent to rationalfit(s.Frequencies,S.Parameters).