Where can I find the definition for National and US National?
United States Code
A good source to use is uscode.house.gov
Title) 8 Ch 12 Sec 1101 (21) (22)
(21) The term “national” means a person owing permanent allegiance to a state (small s state is USA/state of the union).
(22) The term “national of the United States” means (A) a citizen of the United States, or (B) a person who, though not a citizen of the United States, owes permanent allegiance to the United States, the (corp) United States. This is where they capitalize the S in State. The US national is not a US Citizen.
Certificates of Non Citizen Nationality
https://travel.state.gov/content/travel/en/legal/travel-legal-considerations/us-citizenship/Certificates-Non-Citizen-Nationality.html