Suppose you imported XML data into an InDesign document. Suppose that the layout should convey the markup’s semantics: keywords in italics, proper names in small caps, block quotes indented etc.
There are several ways how to map the markup to the layout. But it is important to know: no matter how you’ve mapped it, once the mapping has taken place, markup and layout information may evolve in totally different directions. And this is dangerous. For real-world XML document types and real-world typesetting, mapping is a one-way street, leading from markup to layout, and not the other way round, as we’ll see later. If you trust that after carrying out author corrections, everything that looks like a keyword will be a keyword in the exported XML, you may be proven wrong later. Or the two paragraphs that you see in InDesign are still a single one in XML, because it has been split only visually after import and the markup hasn’t been updated accordingly.