A Priority Bounty was a type of bounty contract that could be taken out on a specific sentient being. By the rules of the Bounty Hunters' Guild, such a bounty required the bounty hunter that had taken the contract to drop all current leads and assignments in order to pursue the specific target. The collecting of any other bounty that the hunter may have taken on was prohibited under Guild rules; even if other targets on a hunter's agenda walked right in front of them. Thus, such contracts could be hugely expensive.
Boba Fett was known to have taken on at least one such Priority Bounty, ordered by Durga Besadii Tai against the t'landa Til Teroenza, who had arranged for the death of Durga's parent, Aruk. By the terms of the bounty he was unable, therefore, to take in Han Solo and Bria Tharen who were also on Ylesia at the time Fett executed the contract on Teroenza since he had yet to deliver the dead cult leader's horn back to Durga.