U - Military science (General)
UA - Armies: Organization, distribution, military situation
UB - Military administration
UC - Maintenance and transportation
UD - Infantry
UE - Cavalry. Armor
UF - Artillery
UG - Military engineering. Air forces
UH - Other services
For more detailed information, visit the Library of Congress Classification Outline.
U - Military science (General)
UA - Armies: Organization, distribution, military situation
UB - Military administration
UC - Maintenance and transportation
UD - Infantry
UE - Cavalry. Armor
UF - Artillery
UG - Military engineering. Air forces
UH - Other services