#2: Any headless process that's scanning, or performing a behind-the-scenes operation. And note, it's window not application. Because an application could reliably stay open without a window… until now.
Mail.
File Transfers.
Staying available for chat.
Rendering process that can run headless. (Handbrake, I think, plus others.)
Applications that appear to quit are essentially suspended; they consume RAM but not CPU and cannot do work in the background.
If an application needs to do work in the background then it is not subject to automatic termination by either behavior #1 or behavior #2 and simply remains running.
This is interesting to me because I've become obsessed with the idea of running a 10W, ARM-based system headless, that serves webpages (i.e. HTTP) with the requested data via my LAN. You can leave something like that on all the time, and have instant-on for your higher power devices with displays and such.
Remember that an application has to support the sudden termination API, and indicate to the OS that it can be safely shutdown, in order to actually have this condition apply.
If you make no changes to your application, then the OS won't touch it. And if you do decide to support sudden termination, then hopefully you would call disableSuddenTermination before starting any background file transfers, renderings, etc.
Mail. File Transfers. Staying available for chat. Rendering process that can run headless. (Handbrake, I think, plus others.)