Priority : It is basically referred to effect on Business Functions
Severity : It is basically referred to effect on reputation of Business
Lets take some assumptions.
High Priority : P1
Medium Priority : P2
Low Priority : P3
High Severity : S1
Medium Severity : S2
Low Severity : S3
Now I'll try to explain it with some good examples to make the better understanding of priority and severity using IRCTC Website.
On IRCTC website, we need to login, then search our trains, after that book the train and make the payment. we also get a message of ticket confirmation. So, we see how the priority are distributed in referance with effect on business.
High Priority (P1) : Login to IRCTC, Searching the Trains, Book a Train, Making the payments
Medium Priority (P2) : Password Reset not working, Cancelling a Train and Modifications in Tickets
Low Priority (P3) : Message of confirm ticket not coming, mail of ticket booking not sent
On IRCTC website, we see to login page, IRCTC Logo. So, we see how the Severity are distributed in referance with effect on business.
High Severity (P1) : IRCTC Logo is not correct,
Medium Severity (P2) : Font sizes are not appropriate and have multiple types.
Low Severity (P3) : Background colour of the page