Incoming call to custom context

I’m a bit confused with Misc destinations and Misc App.

A Misc destination can only be an extension ? This extension would always be custom extension ?

A Misc App is a custom context ? From the FreePBX interface, I don’t get it.

It used to be a simple custom-app field there and you could route an incoming call to a custom context.

How to go about in this version of Free PBX ?

I am trying to route incoming calls to a a2billing context. Also, any tips to integrate a2billing with FreePBX / Trixbox 2.6.

There are guides but I guess they just might not work with the new versions here.