What subjects do the call letters cover?
Subclass U Military science (General).
Subclass UA Armies: Organization, distribution, military situation.
Subclass UB Military administration.
Subclass UC Maintenance and transportation.
Subclass UD Infantry.
Subclass UE Cavalry. Armor.
Subclass UF Artillery.
Subclass UG Military engineering. Air forces.
Subclass UH Other services.