low impedance means the current needed from the ecu to open the injector is high(lower the impedance-higher the current)...(higher the impedance-lower the current).
impedance, flow rate and type of injector needs to be know to accurately determine which injector is needed.
by type of injector i meen whether or not they are (peak then hold) or normal.
some ecu's sre dependant on this type, motecs for example.