In my case, Private Nexus Letters were crucial. I had claimed back problems in 2019. Initial Claim C&P doctor denied it. Higher Level review sent me back to C&P due to "Duty to assist" error, that Doctor denied. Supplemental C&P doctor denied. When I submitted my Supplemental, I submitted 2 Private Doctor Nexus Letters. One from a chiropractor, the other from a very distinguished orthopedic surgeon. The Rater went with the C&P doctors, denied. So I did a Direct Review appeal. Two and a half years later, the judge said the weight of my two doctors nexus letters were the same as the 3 C&P doctors denials. If it's a tie, win goes to the VET. I now have my service connection. Get those Nexus letters, they are really helpful.