When you edit some elements in the forms designer, you can assign external value definitions that use custom vRealize Orchestrator script actions to supply information not directly exposed by the workflow.
For instance, you might want to publish a resource action to install software on a provisioned machine. Instead of providing the consumer with a static list of all software available for download, you can dynamically populate that list with software that is relevant for the machine's operating system, software that the user has not previously installed on the machine, or software that is out of date on the machine and requires an update.
To provide custom dynamic content for your consumer, you create a vRealize Orchestrator script action that retrieves the information you want to display to your consumers. You assign your script action to a field in the form designer as an external value definition. When the resource or service blueprint form is presented to your consumers, the script action retrieves your custom information and displays it to your consumer.
You can use external value definitions to supply default or read-only values, to build boolean expressions, to define constraints, or to provide options for consumers to select from lists, check boxes, and so on.