[M4IF Technotes] Macroblock stuffing

[Tania Orenshtein]

Dear experts,
Can someone explain me the macroblock stuffing issue.
As I understand from the standard, macroblock stuffing can be in the middle
of the VOP (if mbtype is STUFFING). In case of stuffing macroblock, the
macroblock
function returns to the combined_motion_shape_texture, performs some checks
and can go again to the Macroblock(), which may contain again mbtype =
STUFFING and so on
up to the valid macroblock mbtype. Is it true?
The checks at the macroblock end are (COR-2):
combined_motion_shape_texture() {
	do{
		macroblock()
	} while ((nextbits_bytealigned() != resync_marker  &&
	 nextbits_bytealigned() != ‘000 0000 0000 0000 0000 0000’)	 ||
	 valid_stuffing_bits() == 0)
}
There is an explanation of the valid_stuffing_bits() function, which
mentions
valid stuffing bits.
What are this valid stuffing bits values?
Thanks,
Tania