Positioning a GUI on the screen

Ok, this may be trivial but it's bugging me.
I have a gui and I want to have it always be in the top left corner.
Can someone explain what the HorizSizing and VertSizing do exactly?

I've got it to work but it's exactly the opposite of how I thought it would work.

ie I have
- gui control , pos 0 0 , horiz left, vert top, Extent5 12 256
L gui bitmap control, pos 0 0 , horiz RIGHT, vert BOTTOM, Extent 512 256

I would have thought that setting the bitmap control Horiz and Vert to Left & Top would make it go to the top left.... And why is it called Sizing?

Maybe I missed something, it seems that all the tutorials on TDN just assume I should already know how this works...

Thanks :)