Ad inventory management can be defined as a set of actions and analysis aimed at maximizing ad revenue from existing ad inventory.
These operations are performed by ad ops and ad management tools. It’s mainly a technical point of view.
There is also a larger vision which includes more commercial and strategic dimensions, ad inventory management may also include :
sales channels policy (adexchange, in house, premium ad network , horizontal network and often a mix)
ad offering (ad units and placements, targeting criteria)
The last three are not always cited, but are equally important. They are managed by in house sales top management and may be also considered as advertising management.