There's no rhyme or reason to CSS. You can organize it in anyway you want.
For example...Say I wanted to organize #friends, #journal, and #multimedia.
It can be...
#friends {blablablabla;}
#journal {blablablabla;}
#multimedia {blablablabla;}
or...
#journal {blablablabla;}
#multimedia {blablablabla;}
#friends {blablablabla;}
...You get the point. Anyway, there's no organized structure to the ordering of sections. It's advised that one organizes it well, but it's not a law.