I gather you are talking about exhaust noise transfer functions and sound emissions?
Exhaust NTFs are just a way of determining the output sound pressure level divided by the input spl. It is generally shown over the frequency domain. Since most exhaust systems will be used with a number of different engines, it is just a way of comparing A against B. In acoustic modelling the engine source impedance and pulse signature across rpm can be used to determine likely noise output.
The same thing is done with vibrations, although these will not really affect noise emissions. Basically car companies can use the technique to get the right sound for a variety of engine types. It is (potentially) useful in the truck industry, where systems will be compared in terms of system performance and cost.
