The clearer way to describe it is that Fireworks was a reuse of the Macromedia Flash drawing engine. So you had shapes, and these shapes had z-axis positions, a stack of active filters, and a reference to a texture.
A Fireworks PNG document, then, was a pile of shapes (just like a Flash animation frame), with each shape referred to as a "layer"; along with a pile of mutable texture data for the shapes to use, with each texture bound 1:1 to a particular rectangle shape, with the shape and its texture together referred to as a "raster image."
Every time you changed anything, the whole thing just got re-rendered onto a canvas using the Flash rendering logic. When you saved the PNG "document", it kept all the document chunks, but added the baked rendered representation it had been using for previewing as a basic PNG chunk at the end of the document. Thus, it was kind of an actual PNG file. (But the flattened PNG chunk was only "the document" as much as the JPEG cover image inside an .epub is "the book.")
