By papers they mean everything - letters, notes, drafts, communications, etc. It is quite comprehensive and quite interesting including many discussions with other leading researchers key moments in the history of 20th century biological sciences research. It is worth checking out. I note - despite the availability of this great collection, one part of this life as a scientists is not completely freely available - his publications. Not all are at this site and many are hidden behind the walls of various journals. What a shame. Just about every one of his papers is worth reading.