Insertion order
Also called IO.
The signed document that fixes what will run, where, when, at what price, and what happens if it does not.
An IO exists to answer arguments before they happen: dates, formats, volumes, creative deadlines, cancellation terms, and the remedy for underdelivery. Everything in a media plan that is not in the IO is a hope.
The clauses that actually get used are the boring ones — cancellation notice, late-creative policy, and whose measurement settles the invoice. Those three cover most disputes in direct-sold advertising.
How LinkBourse handles it
The order record is the IO: dates, price, deliverable, the acceptance window and the measurement source are all fixed on it when the booking is made, and neither side can change them afterwards without the other agreeing.