A person carrying a standard or flag, usually at sporting events and parades, previously on the battlefield.
A transportation company, such as a shipping or airline company, that is registered in a given state
A strong supporter of a cause, or perhaps the strongest and most visible supporter of said cause.
"Bill was the flag carrier of the movement to improve inner city education."
A national airline (typically having the flag, or other national insignia, on the tail)
An aircraft carrier serving as a flagship