"This sketch shows a regular GoP structure with an I-picture interval of n= 9, and a reference picture interval of m= 3. This example represents a simple encoder that emits a fixed schedule of I-, B-, and P-pictures; this structure can be described as IBBPBBPBB. The example shows an open GoP: B-pictures following the GoP’s last P-picture are permitted to use backward prediction from the I-picture of the following GoP. Such prediction precludes editing of the bitstream between GoPs. A closed GoP permits no such prediction, so the bitstream can be edited between GoPs. Closed GoPs lose some efficiency." - from Digital Video and HD, 2nd Edition, by Charles Poynton.
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