Great news, the Junction Box, demanded by US editors and promised by Waze since Global Champs meeting in Prague 2013 has finally arrived...at least in ß-WME, but visible in production WME too.
Edit: So this is a brief overview for all editors, focused on the visibility in the production WME and not on the (finer details of) application of JB in the beta...

Now how does it work? It is designed to give additional control over turns on a series of junction nodes. Right, guys, as we have on a crossroads of one or more split roads.

First you have to set turn restrictions as appropriate, just considering if the turn from one segment to another is allowed.

Then you apply a junction box, which lets you allow or restrict turns over a series of junction nodes (single node turns are not offered, also directions are taken into account, so do not wonder, if you are not offered all possible combinations, turns against the driving direction or already restrictedturns are automatically filtered out. So, this is an example. I've highlighted point C as an entry point and can define if exit via G or H is allowed or not (yes, exit via G would be the uturn, which we have now perfect under control).
As you can see, junction box does not ask for an exit via F or E (that's behind entry point B), because this is already covered via the turn restriction. Junction Box just goes for the turns involving more than one junction node.
Hope this gives enough understanding how it works and what it's good for.

Attention: Segments within a JB can not be edited (neither beta nor production WME) unless the JB is removed.

Edit: I've been asked to show how a JB looks, if not selected. Well here comes the whole suite, once again, all from the production WME pov: Edit:
Improtant information/confirmation from Mapsir: JB is not affecting routing yet, so don't expect not to be routed through JB-only-restrictions.

Cheers