[Mp4-tech] question about "complentary filed pair" a nd "field_pic_flag"

[ZhouXiao]

Hi all,
I am still confused about the concept of "complentary filed pair".
(1) Why such a concept exists?  Its defination and application seem very amphibolous, Why we have to introduce such a concept? that is: in which condition will a complementary ref field pair or a complementary non-ref field pair occur, and if this condition is true, what special operation will the decoder take? 
(2) Is it possible that the bottom field of one frame and the top field of the following frame immediately in decoding order constitute a complementary ref field pair or a complementary non-ref field pair?
Another question is about field_pic_flag.
As you know, the field_pic_flag is just defined in the slice header layer, 
now supposing frame_mbs_only_flag == 0 && mb_adaptive_frame_field_flag == 0 ( i.e.: using PAFF), is it possible that one slice in a frame adopts field coding mode, while the other slice in the same frame adpotes frame coding mode? 
Maybe it sounds very stupid. and the possible answer is NO. But if that, it seems inappropriate that the field_pic_flag syntax element is put in the slice layer.
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