[M4IF Technotes] Absolute or relative?

[the_ether]

I'm confused about whether motion vectors use absolute or relative
co-ordinates. Being a 'vector' it should be relative but in the ISO doc,
para 7.6 (p203) it states, "Thus the motion vector for a particular feature
inside a VOP, eg a macroblock, refers to the displacement of the feature in
absolute co-ordinates."
On this subject, I have the following questions
1- How are VOP co-ordinates indicated? Is it by the (x,y) co-ordinates of
the top, left corner, relative to (0,0) of the screen?
2- Is (0,0) of the screen the top left corner?
3- Are the co-ordinates of a BAB within a VOP indicated by the (x,y) of top,
left corner relative to (0,0) of the screen?
4- Are motion vectors for a current VOP (and its BABs) relative to the
screen (0,0) or the previous version of the VOP. I.e., if a VOP that appears
in the middle of the screen moves right one pixel, would the MV be (1,0)?
Regards
graham