I think your book is referring to "Driving Point Functions". The driving point function is a sort of transfert function having excitation and response of the network both measured at the same port. If you have, for instance, a two port network, you can inject a current in the port1 (I1) and measure the voltages V1 and V2 at port1 and port2. You can also inject a current in the port2 (I2) and measure the voltages V1 and V2 at port1 and port2. V1/I1 and V2/I2 are the driving point functions of your two port network. In other words they are the input and output impedance. Also the reciprocal function (I/V) is a DP function. Regards