The substitute pennants are used when alphabetic flags or numeric pennants need to be repeated in a signal. The 3rd substitute pennant is used for the first flag in the hoist. It also means the absence of flag officer or unit commander (Inport).
From the International Code of Signals, this long-lasting nautical flag is made from nylon fabric with double-stitched seams, canvas heading with rope, and brass snaphook and ring. This flag is sewn to order.