Hello Christian,
I can recommend you following links:
http://www.unece.org/trade/untdid/d96a/Welcome.html which allows you searching for segments by name (chapter 4.2), e.g. search on BGM, and by clicking on it, you can check all possible qualifiers valid for this segment, so e.g. 383 in your case will be like:
BGM = "Beginning of message"
383 = "Debit note
Document/message for providing debit information to the
relevant party."
you always can crosscheck this in the official VDA 4938 document which can be downloaded on http://www.vda.de/en/publikationen/publikationen_downloads/index.html (search for "global invoic")
Hope this helps!
Good luck!
KR,
Ed