initial
How many units of itself the transporter has to give: one when it is in service, zero when it is not.
Narrowed from Resource, and the narrowing is the point. A transporter is a vehicle: it stands on one zone, and a zone holds one vehicle, so there is no such thing as two units of it. ksl.modeling.spatial.MovableResource narrows the same property for the same reason on the free path.
Without this the inherited control accepted any non-negative integer, and the consequence was worse than an exception would have been. Nothing in the movement machinery reads the capacity -- a pool offers a transporter only while numBusy == 0 -- so a larger capacity changed no journey and no delivery time. What it did change was the utilization statistics, which Resource forms as numBusy / capacity: a capacity of five reported a fifth of the true utilization. A study sweeping this control would have seen flat throughput against utilization falling by exactly 1/capacity and read it as a capacity effect, when it was arithmetic on a fleet that never changed.