What’s the difference between the “mainline” and “stable” branches of nginx?

There seem to be two active branches of the nginx web server: a “mainline” branch (currently 1.9.x) and a “stable” branch (currently 1.8.x). Can anyone provide an official source that describes the difference between these two branches and how one would choose between them?

The difference between mainline and stable is explained here:

https://www.nginx.com/blog/nginx-1-6-1-7-released/

A graphical summary:

Among other differences:

Note that stable does not mean more reliable or more bug-free. In fact, the mainline is generally regarded as more reliable because we port all bug fixes to it, and not just critical fixes as for the stable branch. On the other hand, changes in the stable branch are very unlikely to affect third-party modules. We don’t make the same commitment concerning the mainline, where new features can affect the operation of third-party modules.

Also:

We recommend that in general you deploy the NGINX mainline branch at all times.