The input signal U E for a device (e.g. high response proportional valve) or a system (e.g. control circuit) generally covers a range of U E = 0->U Nominal (max. signal). For high response proportional valves, for example, this would be U E = 0->±10 V.
The small signal range refers to input signals U E < 10 % U Nominal. The term itself is not a standard one.
For a non-linear signal transmitter, the stationary and dynamic transfer function are dependent on the size of the input signal. In the field of electro-hydraulic control technology, the transfer function within the small signal range is of critical importance in most cases. It influences what can be achieved in terms of loop gain and, by implication, the accuracy of feedback control far more than the transfer function in the large signal range.