Designing the information architecture is when I take all the content or features or functional tasks in a product (or website) and organize it into sections that will make the most sense to the end user and be the most streamlined in terms of organization.
Then I go through the laborious task, usually with product marketing folks, of identifying terms that need to be changed so we can create a master taxonomy that will be the standard terms and labels for everyone in the company to use.
I don’t have a singular “information architecture” document, though some people might. Mine have usually been done in the sitemap or wireframe, or a combination of those plus other documents (like a spreadsheet when there is a ton of data involved or a taxonomy list.)
Determining the IA is a critical aspect of any easy-to-use or “intuitive” product. It is especially important for data-intensive products where users have to browse catalogs or search is heavily utilized for meaningful results.
See a ton of diagram examples in this Google search.
Learn more about information architecture and its many definitions.