These are the main scenarios for loading and importing test metadata files:
In the build lab. You are a tester who works in a build lab. Because you are testing compiled code only, you do not have access to the source code you are testing. You might, however, have access to the source files of the tests you are using. As you work, you probably do not have a solution open. To obtain tests to use, load test metadata files into Visual Studio. For more information, see Loading Test Metadata Files.
After you have loaded one metadata file, you might need to run tests in an additional metadata file, In this case, you must import the second metadata file.
During development. You are a developer of production code or of tests. It is likely that your development environment is integrated into Visual Studio Team System. This means that you have access to source control and Team Foundation Build. Eventually, the code you write will either be built in the build lab or used to test production code in the build lab. As you work, you have a solution open. The way you reuse tests is to import existing test projects, import existing tests, or both, into your solution. You can also import test lists by importing test metadata files, but no actual tests are imported when you do this. For more information, see Importing Test Metadata Files.