Media Path Required in Rules Send an SMS to a Number

The Media path value is required when using the Send an SMS to a Number rule. I am not sure what this is and why I would need to populate it. When I send the Test SMS message in the Drupal Twilio Configuration, there is a place to enter the Media Path but it is optional and not required. Shouldn't media path be optional? If not, can you provide some explanation on how to set it? Thanks.