One of the most interesting features of Marshmallow is actually hidden in the OS itself and requires quite a bit of tinkering to get it working. That feature is called multi window (or split screen) and it allowed the user to run two apps side by side on the same screen, similar to how Samsung does it on TouchWiz. However, with the release of Android 6.0 Marshmallow, it remained hidden.
According to a source from site Android Geeks, Google is expected to make this official with the release of the next major point update, Android 6.1. The source also states that the new update will improve the current permission even further to take into consideration apps that require an overlay (similar to Facebook Messenger).
It’s not clear whether this will require a new name or if it will still fall under Marshmallow.