In SDLC, when you feel priority and severity derive the flow?

Once we start creating the test cases with the available requirements, these test cases are executed over the final developed product, while executing these test cases, some of them will pass and some of them will fail. For failed test cases we need to raise a bug and while raising a bug there are few parameters that are defined as a summary of bug, environment on which this bug has occurred, version as well as severity and the priority of the bug as well. So, severity and priority come into picture when a bug is being raised against a failed test case.