Go to the source code of this file.
A chapter contain all the item that the user can handle in his inventory. For example : paper, pen, keys, magasine, tools, books, ladder... The item are common to all location, that's why they are in the chapter.
The last thing a chapter handle is the first location you are when begin the chapter(the changing of location are handled by other way)
So now the xml,
I assume that you know how general xml are formated(tags system like html)
The xml should begin whith a line like that :
<?xml version="1.0"?>
Next there is three types of element possible :
<chapter nbr="2">
<item id="1" texture="icon/ladder.png" combi="-1" combiResult="-1">
<lieu id="0"/>
To see a complete example just look at the source code of this file
The example should create 2 items : a key and a ladder(according to the texture name)
And the chapter should begin at location 3
Definition in file chap.xml.