I had to create several dummy tickets to find out the logic behind how the priority is assigned based on the selection the customer makes in the customer portal. Here is how what we have setup gets assigned. We have 8 priority levels.
P1: Critical (Emergency) - 1 hour
P2: High - 4 hours
P3: Normal - 8 hours
P4: Low - 24 hours
P5: Informational - 40 hours
P6: client Service - 80 hours
P7: Compliance - 100 hours
P8: Project - 480 hours
Here is the matrix for how the selections affect the priority assigned. We have some request types setup to be assigned a certain priority, such as report requests. See table in pic attached.
