When you need more fields for an object than the default three, you can set Part 3 of the object to be an XML field, which then allows you to interact with this field using XML tags, and create many more properties for your objects.
For this tutorial, first create a new data list and define Part 3 as XML. Create an object, and you will see there is only the Part 1 default Title field present, as we would expect.
To add additional XML properties, first click the System Links tab.
This will list all the current system links, notice the TAG icon in the tab menu, click it to view the XML tags.
You will now see a list of XML tags, and a blue plus button, click it to add a new XML field.
Which will bring up a form, complete the form with a Field Name (Shown on the object form), a tag name (Lower case without spaces).
Field Level, which simply effects ordering. Data type, e.g. Plain Text or Integer. Input Entry Method, which defines how you enter the value.
Naturally you should use the appropriate input method, Input Box for text, Drop Down Box for option lists etc.
and finally save your new XML field settings, you will then see it in the list. Back out to the data list children view and try adding a new item. You should see your new field appear on the form.