PDF Formatter - Input Data Example

Are there exampes using the "Input Data" field of the PDF Formatter?  The documentation says:

Input data: String/Expression
Select the specific data from the upstream Snaps that you want to be included in the PDF. If left empty, the entire upstream data source is selected.

In our testing the entire document is written to the resulting PDF even if a specific field name is entered in the "Input data" field.