+@item LYT64MWSNDK34CVYOO7TA6ZCJ3NWI2OUDBBMX2A4QWF34FIRY4DQ.seen
+@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).
+
+@anchor{HdrFile}
+@item LYT64MWSNDK34CVYOO7TA6ZCJ3NWI2OUDBBMX2A4QWF34FIRY4DQ.hdr
+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 @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{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.
+
+@end table