OK, some tips:
- examine the database tables (their content and structure) that the FM is reading
- take the meagre in-line German comments and put them through Google translate
- run a couple of tests in SE37 to see what it does
It makes you weep. A few lines of plain English explaining WHAT the function module does - a few minutes' worth of work - would make all of the above unnecessary. Think of all the function modules in SAP that lack even the most basic documentation... how many people have had to struggle through the same issues?
(OK, this particular FM hasn't been released. But plenty more have).
I'm stepping down off the soapbox now.
cheers
Paul Bakker