Google is introducing a position-preference feature that allows advertise more control over where their ads appear, essentially by specifying in which position they shouldn't appear, according to a post on the SearchEngineWatch blog (via SearchEngineLowdown). The new AdWords feature (not available in the Starter Edition) is located in the Edit Settings screen and allows advertisers to essentially opt out of various positions, acting as a filter.

According to a thread in the SEW Forums, "If you find that your ad gets the best results when it is ranked (for example) third or fourth among all AdWords ads, you can set a position preference for those spots. AdWords will then try to show your ad whenever it is ranked third or fourth, and avoid showing it when it is ranked higher or lower."

Position preferences can be set for any keyword in a campaign. Advertisers can request that an ad appear higher than a specific position (e.g., above 5); lower than a position (e.g., below 4); within a range (e.g., between positions 3 and 9); or a specific position (e.g., position 2). If an ad is ranked higher than a specified position, the AdWords system will attempt to lower the bid to place the ad in the preferred position.

AdWords ranking and relevance rules still determine the ad's position; the preference setting merely determines whether the ad is shown at that position.