A Higher-Level Review (HLR) asks a senior VA reviewer to take a fresh, de novo look at the exact evidence already in your file. You cannot add new evidence in this lane — which is exactly the point. You use it when the evidence was sufficient and the decision was still wrong.
The informal conference
You (or your representative) can request a one-time informal conference with the reviewer. This is where cases are won. It is your chance to point directly at the duty-to-assist error, the ignored favorable evidence, or the misapplied rating criteria.
Difference-of-opinion vs. clear error
An HLR can overturn a decision based on a clear error or a difference of opinion. If the reviewer identifies a duty-to-assist error — for example, the VA never ordered an exam it was required to — the claim goes back for correction with your effective date protected.