Under arms describes a state of military readiness (actual or ceremonial). Typically, troops are considered "under arms" when they are in uniform, on duty, and carrying a weapon (rifle, side-arm, or sword), as opposed to being in uniform, on duty, but not carrying a weapon.
Soldiers not actually carrying a weapon but wearing a service belt or web gear associated with carrying weapons may be considered to be symbolically "under arms".
The leather service belt is the standard sword belt for officers and may be issued to SNCOs and NCOs for ceremonies, parades, honor guards, and reviews when the sword is prescribed, and upon other occasions when an individual is actually or symbolically 'under arms.'
[Headgear s]hould not be worn indoors or in no-cover areas unless under arms
Officers and enlisted men under arms uncover only when ... Seated as a member of (or in attendance on) a court or board [or] Entering places of divine worship [or] In attendance at an official reception.