To determine during a test run on which configuration(s) testing should be performed you can create Configurations.
Configuration groups are found in the Project settings:
Let's imagine that your team is working on a web-application, and you want to test its behavior in several different browsers. For that, you can create a "Browsers" configuration group and list all the browsers you plan to be performing testing in:
Once your configuration group has been created, and you've added all possible individual configurations to test on, you can now specify this while preparing a test run - all available configuration groups will be shown in the "Configurations" section:
You can create multiple configuration groups and have combinations of, for example, different browsers and different OS:
You can also edit or delete both configuration groups and individual configurations after having created them: