A product can be an app.
It can be a book.
It can be a store shelf, a content system, a SaaS tool, a course, a certification path, a template, or a digital download.
The format is not the point.
The point is that the thing becomes clear enough to use, buy, read, follow, or build on.
That is the product line.
This essay is the shell for that lesson. The full version should explain the shift from “software only” to a real product-a-week system.