I am so sorry for my late update because of my study!
A quite update of myself: I have actually stopped working on H.264 and MP4 for some time but will probably pick up again in the near future.
As in the article Introduction to H.264: (1) NAL Unit, Suji Mani asked what is the exact meaning of different values in nal_ref_idc. In fact, nal_ref_idc is base on “Start Code Value” and represents the priority of the current frame (i.e. how important of the frame – the higher the value, the more important the frame). Here is a list of nal_ref_idc values with the corresponding Start Code Type: