OpenOffice.org download options page lists a source archive file named "binfilter".
What is a binfilter?
It has to do with the file formats supported, check here for more information about it: http://framework.openoffice.org/binfilter.html
Is has been obsolete since OOo 1.1 and should be downloaded separately.
From the framework wiki page:
So, in short, it provides the ability to open older binary StarOffice file formats which are now obsolete.