What's an appropriate usage of `@xdn/core/BACKENDS`?

In this guide (Moovweb XDN Documentation - React), there is reference to

import { BACKENDS } from '@xdn/core'
...
  .fallback(({ proxy }) => {
    // send all requests to the server module configured in xdn.config.js
    proxy(BACKENDS.js)
  })

which, according to the comment, is to fallback to the server property in xdn.config.js, but how? I can’t find reference to BACKENDS in the API docs (@xdn/core - v2.37.2) and BACKENDS.js has a value of __js__ which is confusing to me on how this is linked to the server property in xdn.config.js, and even more confusing that it doesn’t reference something within the backends property of xdn.config.js as the name implies.

What are some other uses of referencing @xdn/core/BACKENDS?

Looks like that is the only mention of BACKENDS in the guides. There is an easier way to invoke server side rendering:

  .fallback(({ renderWithApp }) => {
    // send all requests to the server module configured in xdn.config.js
    renderWithApp()
  })

I’ve updated that snippet in the docs.