Premium inventory refers traditionaly to high quality ad inventory, the quality being based on the context, format or targeting.
Premium inventory is ideally sold by in house sales teams according to ratecard conditions and on a guaranteed delivery basis. The term is usually used for well known publishers. It can be also used for high profile ad placement sold trough premium ad network.
Premium inventory is sold at high CPMs through insertion orders which engage publishers and premium ad networks towards agencies and publishers.
Special opérations and some ad units and placements are only avalaible as premium inventory. Other placement or targeting options are initially premium inventory but become remnant when unsold.
Premium inventory may also be used for designate best ad placements on a network.
Premium inventory is often see as a synonym of guaranteed inventory but the concepts are theorically distinct. Premmium or non-premium inventory terms qualify the quality of inventory and guaranteed on non-guaranteed inventory terms qualify the sale process.