[Mp4-tech] [H.264] purpose of non-existing pictures

[Martin.Lange sci-worx.com]

Hello everyone,
a thing which I was always wondering is the purpose of non-existing
pictures. What is a scenario in which an encoder would decide to
intentionally introduce gaps in frame_num, thus producing non-existing
pictures? The only effect I see is that they make other pictures fall
out of the decoded picture buffer and that they also get incorporated
into reference lists, thus changing the reference indices other pictures
would have without their presence. Can this lead to any gain in coding
efficiency? Can anybody comment on what was the intention of this tool?
If this is a question which has already been posted in the past, I would
be happy if anyone could point me to some kind of FAQ of this forum, if
it exists (does it?).
Best regards,
Martin Lange