After "why did it stop", the question a plant asks of its line is "what is it making, and will it be done by two". The order on the line comes first, with its count against its quantity and, once it has run long enough to have a rate, when it will be done at that rate against when it is due.
Start is offered on each queued order and refused, with the reason, while another order is on the line: a line makes one thing at a time. Complete and Abort ask first, because a finished order is not started again. The station loads an order's recipe as it starts it; if the plant will not take all of the recipe, the order does not start.
The queue raises orderAction; this page plays the station that acts on it.
readOnly: the same queue on an office screen, without a button on it.