Here is my guess,
Most likely, you have an attorney and this is why the LUD is specifically indicating that the beneficiary will receive the "approval or registration" document. Based on my experience with attorneys, only the final decision or the physical card like the GC will only be sent to the beneficiary and everything else like the RFI will go directly to the attorney who will relay the info to you. So, based on those facts, this seems like a final decision and is not a request for medical. Furthermore, since the words approval and registration are mentioned, I am again guessing that your father's case has been approved and this deserves my congratulations to you and your father. I personally would be happy and begin celebrating. I'm just like you and am not an attorney and I can be wrong, but this seems like very good news.
Best wishes
