+.BR \-\-exclude=(true|false|flag)
+
+A message is called "excluded" if it matches at least one tag in
+search.tag_exclude that does not appear explicitly in the search terms.
+This option specifies whether to omit excluded messages in the search
+process.
+
+The default value,
+.BR true ,
+prevents excluded messages from matching the search terms.
+
+.B false
+allows excluded messages to match search terms and appear in displayed
+results. Excluded messages are still marked in the relevant outputs.
+
+.B flag
+only has an effect when
+.B --output=summary
+In this case all matching threads are returned but the "match count"
+is the number of matching non-excluded messages in the thread.