List attributes of parallel tests in TestNG?

TestNG allows for tests to run in parallel or in multi-threaded mode. Here, different threads are started simultaneously and test methods are executed in them. The parallel attribute on the 'test' tag will run all the @Test methods in the tag in the same thread and each tag will run in separate thread parallelly. Not only in the 'test' tag, but parallel attribute can also be set in classes, methods, instances to run it parallelly.