Petri net schemata are an intuitive and expressive approach to describe high-level Petri nets. A Petri net schema is a Petri net with edges and transitions inscribed by terms and Boolean expressions, respectively. A concrete high-level net is gained by interpreting the symbols in the inscriptions by a structure. Its semantics can then be described in terms of a transition system. Therefore, the semantics of a Petri net schema can be conceived as a family of transition systems indexed by structures. In this paper we characterize the expressive power of a general version of Petri net schemata. For that purpose we examine families of transition systems in general and characterize the families as generated by Petri net schemata. It turns out that these families of transition systems can be characterized by simple and intuitive requirements.