its supposedly the max stable overclock..
many find it to be a little conservative and have been able to run higher
for me i detects 409/1.11ghz on my 6800gt, but ive found that sometimes even at 400/1.1ghz ii wont get artefacts as such....itll jus pause for like 10 seconds, freeze or mess the image up and lock....quick reboot sorts it.
its supposed to be max stable oc