There’s stuff we do not want to be accessible on the ‘Net, except to our own users. The simplest method for us to effect this is to require the connecting web browser to be running on a computer inside our own network. We now have a site in this here WordPress multi-site that is set up that way.
We may occasionally put a link here (in this Internet-facing site) that points into that “hidden” site — it will be marked up like this link (instead of the standard look …) to alert you to the fact that it may not work, if you are surfing from somewhere else.