That should really end up in the API docs… https://help.mikrotik.com/docs/spaces/ROS/pages/47579160/API#API-Replyword
And, there reports that it breaks the Terraform provider, since it’s Go API library isn’t expecting the !empty:
http://forum.mikrotik.com/t/router-os-7-18-beta-6-terraform-provider-v1-76-3-error-unknown-routeros-reply-word-empty/181984/1