SEC staff review filings and frequently write to issuers asking them to justify an accounting choice, expand a disclosure, or remove a misleading claim. The exchange continues until staff are satisfied, then the whole correspondence is posted on sec-edgar, usually some months later.
For an analyst this is unusually rich material. It shows exactly which numbers the regulator found hard to believe and what the company said when pressed. Revenue recognition, non-GAAP measures, segment definitions and going concern language are perennial themes.
A comment letter is not an accusation of fraud. Most reviews are routine. But repeated letters on the same topic, or a restatement that follows one, deserve attention before you size a position in the name.