In the Test Cases tab in a test run you can add test cases (and test scenarios). When a test case is added then a copy is made from the test case to the test run. This means that a change to the original test case in the Test Design module will not affect the test case in the test run. When you copy a test scenario to a test run then the test cases in the test scenario are copied to the test run.
In a test run, click Modify and, in the Test Cases tab, click the Add function button to open the 'lightbox' for adding test cases and test scenarios.
In the Test Cases tab, you can select test cases and copy them to the test run. In the Test Scenarios tab, you can select test scenarios and copy them to the test run.
Select the test cases (or test scenarios) you want to add and then click the button Copy to test run. The test cases are copied to the test run. To the 
icon for a test case (or a test scenario) you can see that the test case occurs one or more times in the test run.
The test cases are added at the bottom of the list. When you add multiple test cases at once they are added in the order the test cases are sorted in the 'lightbox' Add test cases.
When you add a test scenario then all the test cases in the test scenario are copied to the test run. The test scenario itself is also copied to the test run.
In the Test Cases tab in the test run, the ID column also displays the SCE number if a test case belongs to a test scenario.