This feature is a characteristic of how form fields (widgets) are identified. It is applicable to a form deployed in an external URL.
Each human task (step) has a form which is presented via HTML in a form in a web application (via a URL) when a process case is executed. Each field in the form has a unique ID (name).
You can pass initial values to the form fields (defined by widgets) as parameters using the form URL.
Example: process A can be monitored by process B using a URL. Process B displays information about process A in a form which contains the ProcessInstanceUUID. Process A must pass the argument ProcessInstanceUUID to the URL.
The URL for form instantiation deopends on how a process is deployed. Here is an example: