The Information Architecture (IA) is a written document that describes each page, feature, view, state, and functionality of the system. As you are writing this document, I recommend to describe the features by structure as well as to note the functionality of each area. Every element and it's functionality envisioned on every view should be described insofar as what components make it up as well as how it works. In the best case scenario, this document will also describe every page, every view, every element, every function, and the purposes of all these things within the system. The more time you take to plan this aspect of your project, the faster and smoother the rest of the project will go.