Retainer rings are generally used with internal halyard poles (where the rope for the flag runs on the inside of the pole) as a way of keeping the bottom of the flag close to the pole.  Retainer rings can also be used on large multi-grommet flags for both external and internaly halyard poles as a way of keeping the flags close to the pole to reduce the strain on the upper and lower grommets.

This beaded retainer ring is constructed using white nylon 1" balls and strung onto a 1/8" stainless steel aircraft cable with a stainless steel thimble attached to each end with two copper crimps.  Includes a stainless steel quick link.  Designed for poles with a diameter of 6".  Total length is 34".