Here's a quote from the US Gov:
"The Standard For Unqualified Made In USA Claims
What is the standard for a product to be called Made in USA without qualification?
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 and possessions.
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."
There seems to me to be enough weasel-room there for a good bit difference in actual application.
I was surprised to learn that only automobiles, textiles, wool, and furs are required to have such labels. Outside of those products there is even more potential weasel-room...
Here's the FED site with the info:
http://business.ftc.gov/documents/bus03-complying-made-usa-standard