You can mark packets in the "Packet List" pane. A marked packet will be shown with black background, regardless of the coloring rules set. Marking a packet can be useful to find it later while analyzing in a large capture file.
The packet marks are not stored in the capture file or anywhere else, so all packet marks will be lost if you close the capture file.
You can use packet marking to control the output of packets when saving/exporting/printing. To do so, an option in the packet range is available, see Section 5.8, “The Packet Range frame”.
There are three functions to manipulate the marked state of a packet:
Mark packet (toggle) toggles the marked state of a single packet.
Mark all packets set the mark state of all packets.
Unmark all packets reset the mark state of all packets.
These mark function are available from the "Edit" menu, and the "Mark packet (toggle)" function is also available from the pop-up menu of the "Packet List" pane.