So just what does size zero really mean?

http://www.pro-thinspo.com/size0.html

If you've been wondering like me, just what size zero is, then I've found the answer (well half an answer) - from a reputable source.

A US size zero model is equivalent to a UK size 4. This is where things get a bit "iffy" though - some think that the UK size 4 is equivalent to a size 6 in Australia while others disagree?

Have you got any ideas?