sources.list tokens (section1, section2, section3) help

I'd like to get my sources.list file into a state where I know exactly what apt-get is using, but I don't see an explanation in the apt-get documentation at the debian site about the 3 tokens after the URL.?? It mentions them as section1, section2 and section3, but no more than that.?? The 'stable' token is simple enough, but I've seen things like 'sid', and sometimes there are entries w/ only 1 or 2 tokens, and I've also seen './' used as a single token.?? Can anyone enlighten me or point me to a comprehensive list of these tokens??? Thanks.