The priority matrix helps you to determine the Priority automatically based on the Impact and Urgency of a request. The impact is listed in the y-axis, and Urgency list in the x-axis of the matrix. Priority Matrix requires a one-time configuration by the Administrator.
Once you set the Priority for specific Impact and Urgency in the priority matrix, when a requester selects a similar impact and urgency combination in the new request form, then the priority of the request will be determined based on the matrix.
Go to Setup
>> Customization >> Helpdesk.
Under Priority Matrix, you can find the priority matrix with impact list in the y-axis and the urgency list in the x-axis.
From the Priority Matrix page, click the priority link against specific urgency and impact, this opens the drop-down box.
Select the corresponding priority from the list. The priority will be set for the particular Impact and Urgency combination.

Example:
In the priority matrix: If Impact is Affects Maintenance and Urgency is High then Priority should be High.
Result in request form: If requester/technician selects Affects Business for impact and high for urgency in the request then, the priority field gets filled automatically and displays as High.