You have probably noticed it on some garments that you bought.

It is a number, usually somewhere on the care labels, most likely with the letters RN# before it. As in the picture below.

An RN number or registered identification number is a number issued by the FTC (Federal Trade Commission), upon request, to a business residing in the U.S. and engaged in the manufacture, importing, distribution, or sale of textile, wool, or fur products.