You can pass an argument to your output action with the
$workflow->item()->arg('bob'); // or $workflow->item()->argument('bob');
You can also pass an array to either method:
$workflow->item()->arg([ 'The Roquefort Files Burger', 'Hit Me With Your Best Shallot Burger', ]);
While passing an argument is optional, it's highly recommended that you do. If excluded, you won't know which item the user has selected.