Possible values:
parent: forces headers to use\Hparentargument to specify their levelnumber: forces headers to not use\Hparentargument to specify their level, i.e. to use a number or a number of=
You should basically always set either one of those on any serious project. Forgetting a
parent= in a project that uses parent= everywhere else is a common cause of build bugs, and can be hard to debug without this type of linting enabled.