Registration numbers are provided by the United States Federal Trade Commission to companies that engage "in the manufacture, importing, distribution, or sale of textile, wool, or fur products." This criteria is fairly limiting, and it's further limited by the voluntary nature of applying for a registration number. Companies are not required to get one. For companies that do have registration numbers, the numbers can be readily found through the Federal Trade Commission's online database. Since registration numbers are sometimes used in place of company names, the numbers can be useful when verifying the manufacture of an article of clothing.
Go to the Federal Trade Commission website's RN Query.
Enter the name of the company whose registration number you wish to find. If you do not have the name, you can search using other criteria, such as state or Zip code.
Click the "Find" button. This will bring you to a list of companies that meet your criteria. The company's registration number is in the column under "RN Number."