From thoughts of predecessors about archival science on fonds theory in different periods, three equivalents in fonds have been generalized: fonds equivalent to legal entity files, fonds equivalent to an organic whole of files, fonds equivalent to object fonds. Only with a comprehensive understanding of the three equivalents can the essence of fonds be accurately grasped. In practice, without roundly understanding of the Fonds theory it would be unable to know which is right, how to start, or even commit errors. In today's archives management and network archives and other information resources organization, fonds theory still has methodological significance in guiding the archival information resources integration work.