This template is a LaTeX version of the RUP's vision document for small projects. The MS Word model is available in https://files.defcon.no/RUP/webtmpl/templates/req/vision_informal.htm or https://www.tesestec.com.br/pasteurjr/rup/process/artifact/ar_vsion.htm.
A useful document for game designers to layout your ideas in a way that makes sense to outsiders. It's not 100% necessary to have all of these sections always do what is best for your projects which may need special sections such as 'narrative' or if your project is far more script heavy then look into LaTeX plugins for UML and class diagrams to make your doc extra pretty.