English to Russian Bus/Financial Finance (general)

Здравствуйте, подскажите, пожалуйста, что такое open book execution

Design of oxygen and nitrogen pipe from the AGSS area to the steelwork: Requested flows and design pressures, routing details, details of existing pipe racks/ supports which can be used etc.
Bidder will finally base its offer on a cost plus basis (open book execution plus agreed margin).

Спасибо