What does "marker" mean?

Marker: One who or that which marks. Specifically: (a) One who keeps account of a game played, as of billiards. (b) A counter used in card playing and other games. (c) (Mil.) The soldier who forms the pilot of a wheeling column, or marks the direction of an alignment. (d) An attachment to a sewing machine for marking a line on the fabric by creasing it.