This setting controls the priority of the bookings, 10 being the highest, and 80 being the lowest. Bookings are selected and shown by the system on a placement based on their priorities, so the system will look at a priority 10 booking first, then check at 20, then at 30, etc.
If multiple bookings of different campaigns are placed on the same priority, the eCPM will decide which campaign is shown first, but this of course also depends on the pacing, frequency, geolocation targeting, etc. of the campaigns.
Depending on the Publisher's adserving strategy, multiple booking levels can be in use, each reserved for specific purposes.
As an example, the setup could be the following:
10 - used for sponsorship, also called SOV, campaigns
20-30 - used for direct sold campaigns on CPC or CPM (the bookings can optimize themselves on eCPM)
40-60 - used for demand from the Passendo Exchange
70 - used for house campaigns, potentially optimized on eCPM as well, or at leas on targeting and frequency
80 - used for a 1x1 fallback