We study the spectrum in such a PT −symmetric square well (of a diameter L ≤ ∞)where the "strength of the non-Hermiticity" is controlled by the two parameters, viz., by an imaginary coupling ig and by the distance ℓ < L of its onset from the origin. We solve this problem and confirm that the spectrum is discrete and real in a non-empty interval of g ≤ g 0 (ℓ, L). Surprisingly, a specific distinction between the bound states is found in their asymptotic stability/instability with respect to an unlimited growth of g beyond g 0 (ℓ, L). In our model, all of the low-lying levels remain asymptotically unstable at the small ℓ ≪ L and finite L while only the stable levels survive near ℓ ≈ L < ∞ or in the purely imaginary force limit with 0 < ℓ < L = ∞. In between these two extremes, an unusual and tunable, variable pattern of the interspersed "robust" and "fragile" subspectra of the real levels is obtained.PACS 03.65.Ge, 03.65.