What are "Content Files" (in Visual Studio : Setup Project : File System

In the context of a Visual Studio 2008 Setup Project, what are "Content Files". In other words, when creating a setup project and defining the File System settings and choosing: Add Project Output > Content Files, what files will be added?

For example, what has to be true about a file or its开发者_JAVA技巧 location for it to be considered a Content File for a given project's output?

The build action property of the file will be labeled "content". Here is a link to more information about File Properties.