  We consider the possibility that one extra $Z\equiv Z'$ exists with arbitrary
mass and fermion couplings that do not violate (charged) lepton universality.
We show that, in such a situation, a functional relationship is generated
between the \underline{deviations} from the SM values of three leptonic
observables of two-fermion production at future $e^+e^-$ colliders that is
completely independent of the values of the $Z'$ mass and couplings. This
selects a certain region in the 3-d space of the deviations that is
\underline{characteristic} of the model ($Z'$ "reservation"). As a specific and
relevant example, we show the picture that would emerge at LEP2 under realistic
experimental conditions.
