As a user, I can configure the same issue type as Requirement and Defect

The same issue type can be configured as a Requirement and a Defect.

Think of the incident and problem management process for a software. A customer reports an incident and it goes for detailed analysis in to the problem management. So do the issues being found internally being raised as new problems. For each problem, new test cases needs to be created. So, the issue type problem needs to be mapped as “Requirement” so that the test manager knows which problems are uncovered.

Now think of the testing phase.
The solution for the problem is now implemented. The test case defined for the problem passes the tests but a new problem is identified. According to our process, this needs to be raised as a new problem. So, we have the problem issue type on the one hand as a requirement and on the other hand as a defect. It is however only possible to map an issue type either to requirement or to defect, not to both.