the NetImmerse file format is a proprietery format
whose contents are a closely guarded secret. It appears that companies
that license or buy this engine do not have the right to release its
code. Also, the companies that license the engine can extend the file
format by adding custom object classes to it. This means that even if
you knew the default NetImmerse file format, you would still not be
able to make sense of the nif files shipping with a particular game
unless you knew what modifications had been made to it.