No single record owns these parts of a theme repo, so they are rarely written down. The unit starts with what a visitor sees when something has gone wrong or has just happened: the error pages, and the messages the platform carries across a redirect. It then covers how a template tells the live site from a copy, and how an administrator changes settings and switches features without a deploy, including the structures site data returns. It ends with launch work: the static page shown when the site cannot answer, and what robots rules, the sitemap, canonical links and structured data tell search engines.
If you are preparing a launch, start with the error templates. Read the environment and site data pages together: each one says what the other is for.