I've had this said to me today in #qt-qbs:
"There needs to be a chain of file tags leading to the product type. That's the basis of the entire concept."

Here are the links, that you'd think would be relevant to the concept:
http://doc.qt.io/qbs/artifact-item.html
http://doc.qt.io/qbs/product-item.html
http://doc.qt.io/qbs/rule-item.html

The "concept" is not explained in any of these. Heck, it is not event mentioned. Any example of Product::type: mentioned in the docs only ever uses "application" or other simple type and none use the thing you actually need to understand how to generate code AND application at the same time

type: ["application", filetag]

As a result of the above, anyone trying to use, for example, Rule example, in their own code for a custom file type, will see it not working without any indication of WHY is it not working, neither will they find an answer in the docs.