[NTG-context] What do \start and \stop mean?

I found \start and \stop referenced in the context reference manual (e.g. section 5.5). While \startXYZ - \stopXYZ pairs are discussed earlier, I can't find any place that \start and \stop (with no suffix) are described. They seem to be used like \bgroup and \egroup. Is that right? They are defined in core-sys.mkiv, but I can't figure out from there what they actually do. I'll contribute documentation for them if someone can tell me what they mean. Thanks, Lars