From the in-product Help:
By default, a task is configured to 'fail fast', ie. to immediately terminate the task if any sessions fail to start or terminate due to an exception. This can be overridden by unticking the 'Fail fast on any errors' checkbox.
Note that detecting a task failure works differently when fail fast is enabled compared to when it is not.
If a task is set to fail fast, a task is terminated when any errors in any of the sessions occur - eg. if a resource couldn't be contacted or was too busy to create a session, or if a session raised an exception and terminated abnormally.
If a task is set not to fail fast, a task is only terminated if an error occurs in all of its sessions - and that task termination only takes place when the last running session is detected to have failed.