PEWS^1 is an interface description language for both individual and composed web services. Individual web services are expressions over WSDL-defined operations while composed web services are expressions over PEWS-defined services. This paper distinguishes two dimensions of PEWS. The control dimension describes the control workflow of the service, i.e. the order in which the operations of the web service will be executed. No data constraints are imposed. The data dimension adds information flow to the control dimension. In order to analyse the composability and the substitutability of composed services, we introduce a trace-theoretical presentation of the language constructs. Our long-term goal is to integrate PEWS into a web service design system that checks useful properties of the service specification.