A message tag is a dot separated list of identifiers, specifying the non-hierarchical category for the given message. These non-hierarchical categories are named hierarchically, each part being more and more specific. Examples for message tags are: core.policy or http.request.
Message tags can be used for at least two different purposes:
write them to the syslog, so later analysis may make use of it
filter messages in different categories
In addition to simply writing it to your syslog, you can specify different verbosity levels for different log categories with the --log-spec command line switch.
--log-spec is a list of comma separated clauses. Each clause specifies the verbosity level for a matching set of categories. A clause is in the form messagetag:level, where messagetag is matched left to right, and may contain '*' as a wild-card for each part. Level is the verbosity level for the matching message tag. An example log specification might be: --log-spec=ftp.*:10,core.debug:0
Processing is not stopped on the first match, the last matching clause will be used. If none of them matches, the default verbosity level specified by --verbose will be used.