First, look at the following: Administration Tab->Administration->Service Desk->Request/Incidents/Problems->Priority Calculation
and view the Default priority calculation.
Here you can set default values for Impact and Urgency (and how they can be used to calculate Priority). There is Help On This Window for reference.
You can create different Priority Calculations using different values. See the reference when this might be appropriate.
If this doesn't answer your need, please provide more details on your use cases.
J.W.