There are ten user variables with names $n0 - $n9 that can be
incremented by the add command (see section 13). These can be used
for ``scoring'' messages in various ways. If Exim is configured to run a ``system
filter'' on every message, the values left in these variables are copied into
the variables $sn0 - $sn9 at the end of the system filter, thus making
them available to users' filter files. How these values are used is entirely up
to the individual installation.