In Web-based applications, most user interactions take the form of navigating between Web pages. The structure of the navigation model thus has a strong impact on a Web application's usability. However, specifying a user-friendly navigation model for complex applications can be time-consuming, especially when designing for multiple presentation channels. We therefore present the formal semantics of the dialog flow notation (DFN) that provides constructs for the design of modular navigation models, and especially focus on constructs that reduce the specification redundancy within and between channels, thus reducing the design effort for Web-based user interfaces