Our marine flags are made of durable 100% nylon fabric. These flags feature sewn or embroidered stars, and sewn stripes with a heavy canvas heading and brass grommets.
What does it mean? In US usage the practice, almost certainly obsolete, of flying a blue flag with a white descending diagonal stripe from the starboard yardarm (or spreader) of a pleasure vessel when a guest is on board but the owner is absent. Taken from the Dictionary of Vexillology.
