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It is definitly explained in users guide

From CCE SP 2.70 User´s Guide

page 38
2.15 Pulldown setting
When creating a NTSC video from a film, it is necessary to convert
frame rate from 24 fps to 30fps (60 fields/sec). 3:2 pulldown is a
common method for this convertion. And the 3:2 pulldowned source
will be encoded efficiently and achieve better result applying Inverse
3:2 pulldown.
-When you apply 3:2 pulldown, do not set 00:00:00:00 as a timecode
for the first frame.
-When you apply 3:2 pulldown, Output top field first stream
should be selected. If the source is bottom field first, you should
also confirm that Offset line is 1 or odd number.
-When you output DVD video stream with this option, GOP
sequence should be M=3, N/M=4. To specify GOP sequence,
click Advanced button in the lower right of the encoder setting
window. About Advanced video setting window, see page 43.
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page 45
3.1.2 GOP sequence
You can change the GOP sequence modifying M and N/M. M affects
the number of B picture, and N/M does the number of P picture. You
can see the GOP sequence you set above the setting boxes for M and
N/M. The initial setting is M=3, N/M=5.
- Generally, M=3, N/M=5 is used for NTSC DVD, M=3, N/M=4
for PAL DVD.
- When you create a stream for DVD with applying inverse 3:2
pulldown, M=3, N/M=4 should be set.