Research in design rationale support must confront the fundamental questions of what kinds of design rationale information should be captured, and how rationales can be used to support engineering practice. This chapter examines the kinds of information used in design rationale explanations, relating them to the kinds of computational services that can be provided. Implications for the design of software tools for design rationale support are given. The analysis predicts that the "record and replay" paradigm of structured note-taking tools (electronic notebooks, deliberation notes, decision histories) may be inadequate to the task. Instead, we argue for a generative approach in which design rationale explanations are constructed, in response to information requests, from background knowledge and information captured during design. Support services based on the generative paradigm, such as 324design dependency management and rationale by demonstration, will require more formal integration between the rationale knowledge capture tools and existing engineering software.