The administrative record

Everything an agency relied on becomes the administrative record, and that record is what a court reviews if the rule is challenged. This is why the response-to-comments section of a final rule is often longer than the rule: the agency is building the record that shows it considered the objections.

A comment submitted during the open period is part of that record. One submitted after it closes generally is not, which is the practical reason deadlines matter more than they appear to.