ITIL determines the Priority of an incident as a function of its Impact (its direct effect on business processes) and Urgency (the amount of delay that can be accepted before resolution). Most ITIL-based tools (Remedy, ServiceNow, etc.) handle this conversion internally based on customer definitions.
In some cases you may want to implement this calculation directly within a SharePoint list, or, since this is likely a well-understood concept in the organization, apply a similar calculation to another process. For example, a team using a SharePoint list to accept service requests may define the impact and urgency of those requests to determine a calculated