The "Workflow Output" node is used in a called workflow to define its output value. When one workflow is called by another, the "Workflow Output" node can be used to pass a value back to the caller.
In the called workflow, add a "Workflow Output" node:

Enter or select a variable as the output value. The output value can be of any type, such as a constant (string, number, boolean, date, or custom JSON), or another variable from the workflow.

If multiple "Workflow Output" nodes are added to a called workflow, the value of the last executed "Workflow Output" node will be output when the workflow is called.