Hi,
delta files, are the differences between definitions.
If you SEP client has 30/05/2011 definitions and the manager has 01/06/2011, the manager builds a delta file to give to the client only the differences between old and new definitions.
Delta files are usually very small, less than 1 MB, while the full set of defs is bigger than 100 MB, the advantage for your network is clear.
Regards,