why multiple "StartDocumentLoad" for one single web page?

Hi All,

I am not entire sure if this post fits in this group.  But I am trying
to study and embed the layout engine of firefox.  I started off by
trying to figure out the start function of loading a page, and now I
assume "StartDocumentLoad" is sort of the start function of loading a
page. But I saw this function got called multiple times when loading
one single web page. The more complex the web site is, the more time
it gets
called. I am just wondering if it is true that this function is not
for the entire page, but each separate component (or region) in that
page needs its own StartDocumentLoad?

Thanks,
Yuhao