Americans are Nationals of their nation-states, therefore, State Nationals —- while the Federales, both Territorial (military) and Municipal (civil service) personnel are not acting in the capacity of State Nationals and so, call themselves “Federal Citizens” or “United States Nationals” or just “Nationals”.
They, the military and civil service, are all acting as “Federal citizenry” of one type or another, so when push comes to shove, they call themselves “United States Nationals” or simply “Nationals” when they are off-duty, in the reserves, retired in a Federal State of State, or otherwise inactive.