An empty positioning anchor.
A Marker has no properties of its own beyond those shared ones, in addition it has no Size field. It renders on the canvas as a small fixed anchor glyph rather than a resizable rectangle, and can't be resized by dragging.
Use one wherever you need mark an x/y coordinate, or as a positioned parent for something created dynamically at runtime, like a spot to attach a character image
See Shared Node Properties for common field.