I was reading the page at Communication between widgets—Web AppBuilder for ArcGIS | ArcGIS and thinking how it wasn't very complete.
Then I stumbled across the page at Communication between widgets—Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers which is very similar but with more content.
It seems there are parallel streams of content at Web AppBuilder for ArcGIS | ArcGIS and Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers
Why not just merge them into a single location? Esri documentation is already quite complicated comprehensive so why make it harder on yourselves and us to keep up-to-date?
Cheers,
Steve