Items are added to an order several times over an evening, and each addition goes to the kitchen as its own ticket. When the paper jammed or a ticket went missing, the whole order used to be reprinted, and the kitchen could not tell which part was new.
Now every line records which ticket it went out on. When you reprint, only that ticket prints and the earlier ones do not come out again. The reprinted paper carries a starred reprint stamp at the top, so whoever is at the pass can tell at a glance that it is not a new order.
Lines whose quantity increased or whose note changed are reprinted on the same logic. Because tickets go straight to the printer’s network port, no browser print dialog opens; the paper comes out in the kitchen the moment the waiter sends the order.