Priority is used to tell the system that you have a default set of logic that applies in basic conditions, but under certain specific scenarios you want to use special pricing. The system will evaluate the conditions at the highest priority (closest to priority 1), and if one of those is true for the leg/day/trip the system is computing it will be used. if none are true, then the system will check the next lower priority level for components whose conditions match this leg/day/trip.
For most operators, we recommend that your default scenarios be set to priority 5, and you build your more specific scenarios up from there
Priority in landing fees. you may have a default value at priority 5, values for different countries or groups of countries at priority 4, and values for specific airports at priority 3