User sitemaps are automatically generated by Pure SEO CMS and help people and search engines find your content.
Google XML sitemaps are also generated automatically. You need to place the xml template next to your existing sitemap template, and give it the same name, but then it's automatic. However you will need to register this via Google's webmaster tool.
This is more important when you have large sites made up of multiple project as larger sitemaps are then generated linking them all together.
Pure SEO-CMS will automatically create and error404.html page when you build the complete site. This will only be done if no page already exists. An error404 page will automatically be displayed if someone mistypes the page name for your site and no page with that name is available.
It's good to create your own page because some host use their own error pages and include advertising links to other sites. The information for this page is taken from an error404_template page located in the default directory and populated with the information you type into the website information section of Pure SEO-CMS's setup screen.
At the same time the error404 page is created an index.html pages is written into each directory of your website. This page uses the same template and information as the error page and is designed to stop people accessing the directory to look for files. If it was not there they could type in the directory address and see exactly what information or files you had stored there and then access them.