Chapter 8 refers literally to chapter 8 of the Department of Transport Documentation Traffic Signs Manual. It details the requirements for signage, markings and safety guidelines for temporary traffic management situations on highways.
In relation to chapter 8 vehicle chevrons, the documentation details the specific requirements for any vehicles stopping on high speed roads (50mph+). Whilst the Traffic Signs Manual is not legislation, however Class R3B chevrons are a requirement for any vehicle stopping on high speed roads in Scotland & Wales and many authorities will not allow vehicles that do not comply onsite.