unit

What this lever is measured in — "staff", "units", "$/hour". Optional by design (§4.2.4, G.9 row 7): requiring it would be ceremony, and the library cannot verify a unit against the model. What it does with one is check that a joint constraint does not sum levers measured in different things, and name it in every violation message — and, through this descriptor, let a ShapeAwarePolicyIfc refuse a surface whose units are not what the rule was written for.