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?

$ make progquotes
cc -O2 -pipe    -o progquotes progquotes.c
$ ./progquotes ONE WORD THE FORCED INDENTATION OF THE CODE THREAD OVER ! ! ! ! ! ! ! ! ! ! ! ! ! xdddd
ONE
WORD
THE
FORCED
INDENTATION
OF
THE
CODE
THREAD
OVER
!
!
!
!
!
!
!
!
!
!
!
!
!
xdddd


#include <string.h>
#include <stdio.h>

int main(int argc, char **argv){
    int i;

    for(i = 1; i < argc; ++i){
        printf("> %s[o]\n", argv[i]);
    }
    for(--i; i > 0; --i){
        printf("[/o]");
    }
    printf("\n");
    return 0;
}