Explaining the BBcode?

I don't quite think lurk moar would cover this, considering the BBcodes are interpreted and then we only see their output. I understand [u], [i], [b], [spoiler], and [code]. I've seen, but I don't know if [sup] and [sub] work. I have just a few questions.
How do you overline text?
Is there an order to the tags, or can they all be ordered in anyway, like '[i][u][b]' , '[b][u][i]' , '[u][b][i]' , etc.?
To do multilayered spoilers do you simply layer the tag? I saw this one spoilered text that was A-Z and if you highlighted A, it was only A, and B was both A and B, and highlighting Z showed the whole text.
How do you make the text that has the preceeding |, but the background changes color slightly when you hover over it?

test
[b]    bold
[o]    overline
[u]    underline
[i]    italic
[s]    strikethrough
[code]    preserves syntax for code and colors text
[spoiler]    for spoilers
[sup]    makes text progressivly smaller ^higher on the line
[sub]    makes text progressivly smaller _lower on the line
[aa]    For SJIS art.