What's the difference between the top volume bar (squiggle) and bottom volume bar (bell)?
I think they both have something to do with notification volume, and most of the time changing one also changes the other. I am running Cyanogenmod 13 (but I don't think it is a Cyanogenmod-specific feature).
It appears from the screenshot that a theme is in place with its own set of icons. Stock Android Marshmallow doesn't show four but only three bars. The extra one CM shows is for notification volume. Anyhow, a theme alters the icon but not the order of bars. You can always revert to stock theme or icons and get this result:
(Click an image to enlarge it)
If you don't intend to revert the theme icons, then head over to Settings app → Sounds and behold the meaning of icons yourself:
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