The fs keyword argument goes into the constructor of the responsetype object, e.g.
julia> digitalfilter(Lowpass(0.3; fs=1000), Butterworth(3))
ZeroPoleGain{:z, ComplexF64, ComplexF64, Float64}(ComplexF64[-1.0 + 0.0im, -1.0 + 0.0im, -1.0 + 0.0im], ComplexF64[0.9990566353334952 + 0.0016308813925406407im, 0.9990566353334952 - 0.0016308813925406407im, 0.9981168187068135 + 0.0im], 8.355936714097361e-10)