Where is the API?
SmartBear provides JavaDocs here
Unfortuallly the descriptions are lacking in many places.
There appears to be multiple objects that point to these variables. Which one is used is contextual to which type of request you use.
TestRunner is an interface and there are actually several objects it could be depending on the context you are using it in. e.g. SecurityTestRunner, WsdlTestCaseRunner
There is also some useful samples of common usages of scripting here
Hopefully that helps