Proposal to clarify the purpose of UN R139 which was originally established through the extraction of brake assist system (BAS) provisions from UN R13-H on brake systems. Within the context of UN R13-H, the BAS provisions were written to assess performance in cases where a vehicle was fitted with a brake assist system. As a stand-alone regulation, UN R139 applies only to vehicles that are equipped with BAS. Therefore, this supplement proposes to refine the text to remove the “if fitted” language and ensure that UN R139 can only be applied to a vehicle equipped with a BAS.