When you generate a Document using PDFMonkey, you will usually send data to use with the Template you selected.
A Template's test data are fake data that look like the ones you would use when generating a Document. Their purpose is to help you build the Template and test different scenarios, according to the data that would be sent when generating a Document.
It's usually more efficient to play with test data than to build your Template and test it by generating test Documents. You get a shorter feedback loop and don't consume your monthly quota.
As a commodity, your test data is copied as Document payload when generating a Document via the dashboard. Aside from that, it is never used when generating Documents through the API.
The preview is a real PDF
When building your Template, the preview you see on the right of the editor shows you a real PDF. This means that if it looks nice in the Preview, it will look the same when generating a Document.