A major drawback of the set-theoretic solid model is the global and non-unique way in which it is defined. In a recent paper, Rossignac and H.B. Voelcker introduced the concept of the active zone of primitive or submodel, which provides a localization of a set-theoretic model which is useful for a number of purposes. We present a more general, but simplified, definition of the active zone which exploits the properties of the symmetric difference operator. We also outline a novel application of the active zone to the interactive graphical editing of set-theoretic solid models.