The 'Priority' setting can be used to prioritize certain campaigns over others, for example setting paid campaigns higher than internal promotional messages (trailers, etc).
All Campaigns are assigned Standard priority by default, but they can be changed to Urgent, High, Low, or Very Low. Outside of exclusions that disqualify a campaign from being delivered, campaign priority is the first setting considered for selection.
A higher priority campaign will always be selected over a lower priority campaign.