Traditional approaches to plan representation focused on the generation of a sequence of actions and orderings. Richer models, which incorporate plan rationale, provide benefits to the planning process in a number of ways. Rationale is examined in terms of causality, dependencies, and decisions. Each dimension addresses practical issues in the planning process and adds value to the resultant plan. The contribution of this paper is to explore this categorisation and to motivate the need to explicitly record and represent plan rationale where situated, mixed-initiative planning systems are present.