What HLR is for
Higher-Level Review is generally used when you believe VA made an error based on the evidence already in the file. It is not the lane for submitting new evidence.
Evidence rule
The HLR reviewer looks at the existing record. If you have new and relevant evidence, a Supplemental Claim may be the better lane to compare.
Preparation notes
Organize the decision letter, list the exact issue, identify the likely error, and prepare a concise explanation for an informal conference if you request one.