1) Publishers allocate their display inventory between direct "reserved" sales to advertisers and indirect "unreserved" sales through third parties like ad networks and exchanges. In 2011, publishers worldwide sold more impressions through unreserved channels than reserved channels.
2) Sell-through rates, or the percentage of reserved inventory sold directly by publishers, tend to be higher at the end of the year due to seasonal advertiser demand. The gap between reserved and unreserved inventory narrowed over the course of 2011.
3) Channel mix, the ratio of reserved to unreserved impressions, remained relatively steady for APAC publishers but shifted more for EMEA publishers towards the end of the year.