. For a software engineer, it is the map used to navigate the millions of lines of code within the Android Open Source Project (AOSP)
Currently, Android's battery usage screen ( Settings > Battery ) tells you how much battery an app used, but not why or how efficiently it used it. An app might use 1% battery, but if it did so by waking the device up 500 times in the background for no reason, it is behaving poorly. Users and developers lack a quick "efficiency" metric. xref aosp