A more thorough explanation:

The Acceptable Identification of Goods and Services Manual is a guide published by the U.S. Government that provides a list of acceptable international class categorizations and descriptions for goods and services. This guide is used when submitting trademark applications to the U.S. Patent and Trademark Office.

For example, if you were applying for a trademark for a clothing brand, you would use the manual to find the appropriate international class categorization and description for clothing. This helps ensure that your trademark application is properly categorized and described, making it easier for others to search for and identify your brand.

The manual can be obtained from the U.S. Government Printing Office or through the PTO's website.