It is often useful to associate reference material with a heading or subtree. Small chunks of plain text can simply be stored in the subtree of a project. Hyperlinks (see Hyperlinks) can establish associations with files that live elsewhere on a local, or even remote, computer, like emails or source code files belonging to a project.
Another method is attachments, which are files located in a directory belonging to heading/subtree. Org uses directories named either by the ‘ID’ property of a heading or by a ‘DIR’ property.