Testlabs is where you review results and run your testscripts.
Results are always based on the newest run, and a 'parent' will inherit the worst results from it's children, meaning that if a subrequirement has any failed testcases the parent requirement will be marked as failed.
Entire requirements can be run. This will run all it's linked testcases in all available OS/browser combinations. This will not run any sub-requirements. eg. A requirement has 3 defined OS/browser combinations: Vista/FF3, XP/FF2 and Vista/IE7 and 3 linked testcases. There are however no nodes online running XP, so the 3 testcases will each be run in Vista/FF3 and Vista/IE7; 6 runs in total.
Single Testcases can be run, which will run the testcase in all available OS/browser combinations. Note that it's the most immediate parent requirement that defines what OS/browser combinations the testcase should be run in. Eg. requirement 1 has subrequirement 1.1. requirement 1 has the following OS/browser combinations defined: Vista/FF3, Vista/IE7, but 1.1 only has: Vista/FF3. Any testcases linked to requirement 1.1 will only be run in Vista/FF3