Booked orders allocation is a means of ad revenue optimization by which is decided on which ad placements an order of insertion (OI) must be delivered.
For some OI there is no choice. For channel targeting or contextual targeting the ad must appear on specific sections or contexts.
For behavioral, technographic and some demographic targeting, it may be totally different. A sport addict or a user registered as man from 20 to 30 can be exposed everywhere on the site or network. Thus, if publishers have leeway, their best interest is to deliver the ad to the target on a "low value" section or context. The same principle is applicable if the target is a Mozilla Firefox user (technographic criteria).
For big publishers and ad networks the procedure must be automated through algorithms.
Optimized IO allocation is an element of ad inventory management.