I was able to go to the IBM InfoCenter (not a very friendly place) and search with RLU. I then downloaded a PDF manual for it.
All of the ADTS Programming manuals can be downloaded as PDFs, including the ADTS for AS/400: Report Layout Utility.
I tend to agree that RLU isn’t very good. It can, however, create a very useful set of DDS specs for an externally-described printer file. I wouldn’t use it much after creating my first cut nor would I use it for fine tuning column positions or attributes. But it is very useful as a learning tool.
It helps to show how multiple heading lines can be included in a single record format for example. Using it purely as a general page design guide and for first approximation is much better than using it for small details like you would with SDA for display files.