Federal Trade explains the standard as follows:
What is the standard for a product to be called Made in USA
For a product to be called Made in USA, or claimed to be of domestic
origin without qualifications or limits on the claim, the product
must be "all or virtually all" made in the U.S. The term "United
States," as referred to in the Enforcement Policy Statement, includes
the 50 states, the District of Columbia, and the U.S. territories
What does "all or virtually all" mean?
"All or virtually all" means that all significant parts and processing
that go into the product must be of U.S. origin. That is, the
product should contain no — or negligible — foreign content.
For more consumer information on the "Made in USA" Standard
you have any questions or concerns, please feel free to contact
us at firstname.lastname@example.org
with confidence -- buy American!