+.IP \fBpkt_mark=\fIvalue\fR[\fB/\fImask\fR]
+Matches packet metadata mark \fIvalue\fR either exactly or with optional
+\fImask\fR. The mark is associated data that may be passed into other
+system components in order to facilitate interaction between subsystems.
+On Linux this corresponds to the skb mark but the exact implementation is
+platform-dependent.
+.