This provision was designed to protect confidential commercial information as defined in the Freedom of Information Act (FOIA). As a result of FOIA, requested information must be disclosed unless it falls under one of nine exemptions which protect personal privacy, national security, and law enforcement. You can learn more about the FOIA here: https://www.foia.gov/index.html
As a part of your submission, an explanation for the confidentiality request must be submitted to PHMSA along with a copy of the documentation with the confidential information removed. PHMSA will make a decision on whether or not to treat your information as confidential and notify you, in writing at least five days before the information is publicly disclosed.