[M4IF Technotes] I-block vs P-block

[the_ether]

When signalling to the decoder what type of encoding was used, can I have
just one or two blocks / macroblocks that are coded in intra mode whilst the
rest are in inter (P) mode?
How do I signal this? Do I specify that the VOP is a P-VOP and then the
decoder assumes all MBs are of type P unless otherwise specified?
To what resolution does this work? Can I specify the mode down to the block
level or is it just down to the macroblock level? (ie I specify the VOP as
type P-VOP and then within that just one block (or only macroblock?) is
specified as being of type I-block (or only I-macroblock?)?
Thanks in advance for your help.
graham