The Five Planes
The Structure Plane
In the structure plane, a conceptual structure for the Web site is developed. Defining the structure shifts the concern from strategy and scope to factors that will determine what users finally experience.
Interaction design is concerned with describing possible user behavior and defining how the system will accomodate and respond to that behavior. Conceptual models help build users' impressions of how the interactive components should behave. The conceptual model allows you, as the designer, to make consistent design decisions that follow the strategy and scope of your product.
Another part of the structure plane is error handling. This is where you should figure out areas of the product that can be altered in order to prevent people from making mistakes, or building the system to prevent users from making mistakes.