+@cindex seen files
+@item seen/LYT64MWSNDK34CVYOO7TA6ZCJ3NWI2OUDBBMX2A4QWF34FIRY4DQ
+@command{@ref{nncp-toss}} utility can be invoked with @option{-seen}
+option, leading to creation of @file{seen/} files, telling that the file
+with specified hash has already been processed before. It could be
+useful when there are use-cases where multiple ways of packets transfer
+available and there is possibility of duplicates reception. You have to
+manually remove them, when you do not need them (probably because they
+are expired).
+
+@cindex hdr files
+@anchor{HdrFile}
+@item hdr/LYT64MWSNDK34CVYOO7TA6ZCJ3NWI2OUDBBMX2A4QWF34FIRY4DQ
+If no @ref{CfgNoHdr, nohdr} option is enabled in configuration file,
+then @file{hdr/} files are automatically created for every ordinary
+(fully received and checksummed) packet. It literally contains just the
+header of the corresponding packet. It will be automatically created
+even during simple @command{@ref{nncp-stat}} call. On filesystems with
+big blocksize (ZFS for example) it can greatly help listing the packets
+in directories, because it prevents unnecessary read-amplification. On
+other filesystems probably it won't help at all, or even harm
+performance.
+
+There is a hack: you can create more dense @file{hdr/} allocation by
+removing all @file{hdr/} files and then running @command{@ref{nncp-stat}},
+that will recreate them. In many cases many @file{hdr/} files will be
+allocated more or less linearly on the disk, decreasing listing time
+even more.
+
+@cindex ack files
+@anchor{ACKFile}
+@item tx/ack/LYT64MWSNDK34CVYOO7TA6ZCJ3NWI2OUDBBMX2A4QWF34FIRY4DQ
+If tossing was performed with @option{-gen-ack} option, then for each
+generated @ref{ACK} packet there will be corresponding empty @file{ack/}
+file. ACK outbound packets needs to be deleted after the transmission,
+but since they are stored encrypted in the spool, there needs to be some
+kind of knowledge what packets are ACK ones. Both ACK and corresponding
+@file{ack/} files are removed by @command{@ref{nncp-rm} -ack} command.
+
+@end table