Dear Hans-Jürgen,
> could you venture a definition of the term "document" in this context?
There is a distinction commonly made in XML: data-oriented vs. document-oriented (even though I am not sure if this terminology is settled).
A data-oriented XML document looks like records, often with repeating patterns, exactly like in your own example:
<getFlights>
<departureAirport>CGN</departureAirport>
<departureDate>2017-08-18</departureDate>
</getFlights>
(further data can be nested as well). Such documents are easy to map to and from tables, for example.
A document-oriented XML document looks more like an HTML page with mixed content, XHTML and docbook being two prominent examples:
<paragraph>This is <bold>bold</bold> text and this is <italic>italic</italic> text</paragraph>
The former is similar in spirit to JSON -- this is where they "overlap" -- while the latter is one of XML's sweet spots, which explains why it is so popular in the publishing industry.
Kind regards,
Ghislain
