I read it as each unit loses 1 uhp/day, as the game doesn’t seem to operate at the individual-NPC scope; but it could do with a little clarification.
This is not “damage” as such but represents the deleterious effects of hunger, minor disease, and nominal wear-and-tear on equipment that full supplies would otherwise alleviate. Units reduced to 0 uhp, I imagine, wouldn’t die off (they can survive plenty longer) but would be physically unfit for battle. This is to say nothing of their likely loyalty to their officers.
Given that most units have 4-8 uhp, after a few days without supply, an army will have to resort to requisition/looting to maintain its regulars’ effectiveness. These supplies could perhaps be rationed out (per the text on “rations every other day”) to prolong an army’s operation in the field.
That is kinda what I expected, but it does leave a few questions pertinent to our situation.
Can an army that is reduced to 0 uhp march? Do they simply rout when attacked? Does being reduced to 0 uhp trigger a calamity roll for that unit (aside from the weekly roll due to having inadequate supply)?