One of the big new features that you'll find in Android 6.0 Marshmallow is Doze. And essentially what this feature is, it allows your phone or tablet to go into a deep sleep. Instead of being woken up whenever there's a new Gmail message or text message, etc. It's proven to extend the battery life on older Nexus devices, like the Nexus 6 and Nexus 9, for quite some time. Almost doubling the battery on the original Nexus 5. This is definitely one of those "under the hood" features you'll want on a device, whether it's a Nexus or not. Google, is actually mandating that OEM's include Doze in their Android 6.0 devices and updates. Not only do they have to be included, but they must be unmodified. Which is great news. As it should still work the same as it does on Nexus devices. Here's the excerpt from Google's Android Compatibility Definition Document (or CDD):
"All apps exempted from App Standby and/or Doze mode MUST be made visible to the end user. Further, the triggering, maintenance, wakeup algorithms and the use of Global system settings of these power-saving modes MUST not deviate from the Android Open Source Project."
One of the features of Doze is being able to exempt some apps from the Doze feature. So something like the dialer or your text message app, so that if someone needs to get in contact with you, they can. Even when Doze is extending your standby battery life. You can exempt just about any app that you wish to do so. Additionally, these do need to be visible to the user. So they can be in the settings, but cannot be hidden in some secret menu or even under developer options. That's a great move by Google there.
The Android Compatibility Definition Document outlines items that are strongly recommended as well as items that are mandatory. This Doze feature is a "must" (and in all capitals at that), so this is something that all manufacturers will need to adhere to in order to get their device certified for Google Mobile Services or GMS, and without that certification, that means no Google services at all.