   system. Instead, you start one or more applications, each with their own
   initialization and termination logic.
 
+  Applications also provide an "application environment", which provides one
+  mechanism for configuring long running applications. We will learn more about
+  the tooling, start and shutdown and the application environment in the next
+  sections.
+
+  ## Start and shutdown
+
   Starting an application is done via the "application module callback", which
   is a module that defines the `start/2` function. The `start/2` function should
   then start a supervisor, which is often called as the top-level supervisor, since

  shutting down, all applications shut down their top-level supervisor, which
  terminates children in the opposite order they are started.

  We have mentioned the Mix build tool is responsible for compiling applications,
  but it is also capable of running applications. For example, `mix test`
  automatically starts your application dependencies and your application itself
  before your test runs. `mix run --no-halt` also boots your current project and
  can be used to start a long running system. See `mix help run`.

  Developers can also use tools like [Distillery](https://github.com/bitwalker/distillery)

  when tools must be shared between developers and not as deployment options.
  See `mix help archive.build` and `mix help escript.build` for more detail.

  ## Application environment

  Once an application is started, OTP provides an application environment
