What exactly is StimulusOnsetTime (Flip return value)

1
Hi all

I want to include some error handling code that checks to make sure
everything was drawn when I told it to, so I am using Flip's
timestamps. My slightly silly question for the day: in a call to
Screen('Flip'), what exactly is StimulusOnsetTime a measure of? It
looks like StimulusOnsetTime is a tiny fraction of a second after
FlipTimeStamp. So what exactly is it a measure of and why is it
different from FlipTimeStamp?

Thanks
-andrew