Adobe Illustrator - Expand Appearance, Expand and outline Stroke

A question that gets asked many time in Illustrator is What is the Expand Appearance, Expand and Outline each do. Why is it there? What is each used for and why are some of the options not available?

Here's an explainer: (is that a word?)

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When you draw a line, you can add a stroke weight and color to it (above).

In Preview mode (COMMAND + Y) it looks like a single, thin line. 

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When an effect is added to the line, in this case Effect > Distort & Transform > ZigZag, you can see the straight line as the path with the effect of the zigzag effect applied to it. But in Preview Mode (COMMAND + Y) it is still just a line. The effect is just that, an effect and the base line does not reflect the effect. 
You can see the blue highlighted path does not match the appearance of the line.

This is how the ZigZag line looks to Illustrator. The effect is applied but in Preview, its just a line (right).

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When you select the line with the Effect on it, then choose Object > Expand Appearance (above), it will convert the line into the actual shape of the effect. (Right).
​It is no longer a straight line, the line now matches the appearance that you see, thus the term Expand Appearance.
Keep in mind it is still a line. The stroke weight and color can be changed easily and edited and end caps applied.

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At this point the Expand Appearance in the Object menu is now grayed-out and no longer an option. 
​But the Expand is an option. (below)

Since the Effect or "Appearance" has been expanded and the line in Preview Mode matches what you see on screen, the only option left is Expand.
​Expand will take the line, convert it to a shape and it will no longer be editable as a line. (Right)

The Expand feature works the same (on a line or path) as the Object > Path > Outline Path.
​ It takes the line and outline it into an editable shape as shown by the blue highlighted path.

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Here is a quick over view of the 4 states; shown in Normal View Mode and then in Preview Mode to compare.
1: Line
2: Line with ZigZag Effect applied
3: Line with the Expand Appearance applied
​4: Final Expanded or outlined Line.

That should shed some light on the matter of Expand Appearance, Effects, Expand and outline commands.

Happy Illustrating.