We define impedance as the complex ratio of potential (voltage) over flow (current). We can describe it as how much a system "impedes" the flow according to a potential applied to it. But wouldn't it be better to think the other way around, as the resulting potential produced by a flow through the system?
This interpretation of impedance feels more organic with the topology of electronic systems. Once current flows through an element, it produces a voltage potential. The input is current, the output is voltage. This would turn flow into the input and potential into the output. Output/Input = Transfer Function.
This approach makes the concept of admittance, which is sometimes better applicable for mechanical systems, easier to manipulate. The input for a mechanical system is usually force and the output is velocity. Again, Output/Input = Transfer function.
I'm modelling transducers and it seems like transfer functions are a more general concept for describing the behaviour of a system that converts energy between domains (electrical<->mechanical).