When header and trailer are added to pdu?

A header board, is a vertical structure on the front of flat-bed trailers. Its purpose is to keep the freight from moving forward onto the cab. It is humorusly called a headache rack (it prevents the load from shifting forward therefore prevents the driver from getting a "headache" when the cab is crushed.

The terminology is different for a a dry-van trailer or a refrigerated trailer, or what is sometimes called a "box" trailer, wherein the front is called a bulkhead. Sometimes these two terms are incorrectly interchanged by drivers who lack trucking knowledge.