any standard of codec_data ?

hi, gstreamer exports

Is there any document describe codec_data, a field of private data.

In faad, codec_data contains some parameters handled by faacDecInit2.
In mpeg4dec, codec_data could still used to initialize mpeg4 codec,
But for h264dec, it seems it could not work.

 

since there are so many codec, it would be greate if a document regulate this private field.
is there any, where to get it.

 

Thanks