The quick distinction
Higher-Level Review is for a fresh review of the same evidence. Supplemental Claim is for when you have new and relevant evidence.
Choose HLR when
You believe the existing file already supports the outcome and the issue is an adjudication or review error, not missing evidence.
Choose Supplemental when
You can add evidence that directly addresses the reason for denial, the rating level, the effective date issue, or another missing part of the record.
If both seem possible
Preserve deadlines and consider asking an accredited VSO or attorney to review the decision letter and evidence plan before filing.