Jens,
The main reason we haven't put a lot of documentation around the data layer is because we don't require any specific standard or format. Anything that's in the DOM (HTML, CSS, or Javascript) can be referenced in DTM and used to populate data elements.
For example, let's say you had a Javascript object defined on the page that was populated by Adobe Experience Manager or whatever content management system you happen to be using:
var pageData = { "pageName" : "Creating a data layer in DTM", "pageType" : "article", "channel" : "tips and tricks" // etc... };
You can then use the Data Elements feature in DTM to map these key/value pairs to individual data elements. Example:
[img]data_element_example.png[/img]
Hope this helps!
-Shawn