A code-share agreement is an arrangement where two or more airlines share the same flight. In these arrangements, a U.S. air carrier may sell a seat on a flight that is actually operated by a foreign air carrier under a different flight number or code. However, compliance with the Fly America Act is satisfied only when the ticket or documentation for an electronic ticket identifies the U.S.-flag air carrier's designator code and flight number.
See the example below, where Delta Airlines has a code-share agreement with Air France.
Compliant: Delta Airlines (DL) 8399, shown as DL8399 on the ticket, e-ticket, or boarding pass.

Non-Compliant: Air France (AF) 0337, shown as AF0337 on the ticket, e-ticket, or boarding pass.

Documentation Requirements
To satisfy regulatory agreements that a flight is FAA compliant under a code-share agreement, travelers must retain documentation that confirms the U.S.-flag air carrier's designator code and flight number. This information is located on the ticket, e-ticket, or boarding pass. The traveler is responsible for retaining the appropriate document as proof of compliance.
Below is a list of the U.S.-flag air carriers and their codes:
View the FY17 Domestic & International Code Shares (retrieved from the U.S. General Services Administration travel website).