Chances are that if you're in the recovery interface of your phone, it's because you broke something, need to force an update, and you're just fiddling with things. The stock recovery doesn't have a ton of options, which is why alternatives like ClockworkMod and TWRP exist. Still, the stock recovery in Android M is a little more capable than before.

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There appear to be three new options in the recovery menu: apply update from SD card, mount /system, and view recovery logs. Applying an update from SD would be an alternative to using ADB sideloading (which is still offered). The Nexus 5 fails to find the SD card path, as you might expect. This might be intended mainly for Android One devices. It's not clear if this option is going to apply to internal storage (the virtual /sdcard directory), but this is a preview so there's no telling if the error message is even accurate.

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Recovery also allows to mount the system partition for some reason. I can't think of a good reason for this, but I'm sure it has something to do with debugging. Finally, there's a new option to view recovery logs. It will list the last few log files (eg. last_log.1) in this interface so you can choose between them. When open, the volume toggle scrolls up and down.

The text at the top is also yellow now, and it appears to be red on all the current recovery interfaces. This should all be included in the final M rollout for Nexus devices, but for now it's preview only.

Recovery logs

It looks like the recovery logs are available in stock 5.1 too, but the text is yellow instead of white.