CallFunction() ... What are you?

Continuing working through SWFUpload code, I've come across

returnString = movieElement.CallFunction('' + __flash__argumentsToXML(argumentArray, 0) + '');

Now, I think I get the gist of what is happening here - CallFunction is an Active-X method which seems to take an XML style argument in order to call a function in the Actionscript code. However, googling around I seem to be completely unable to find an API or any definition of this method. Can anyone help me out with this?