Commands controlling execution operate on the current focus, or target — either an individual thread or process, or a group of threads and processes. You can individually stop, start, step, and examine any thread or process, or perform these actions on a group.

Select the target of your action from the Group (Control) dropdown menu

You can also synchronize execution across threads or processes using a barrierpoint, which holds any threads or processes in a group until each reaches a particular point.