func_detail

func_detail is a internal brush entity available in all

Source games. It is not an actual brush entity, but rather moves all contained brushes to worldspawn and flags them as CONTENTS_DETAIL, resulting in them not affecting visibility or chopping non-detail brushes. All brushwork that does not form the 'backbone' of the world (and that is not tied to a real entity) should be detail, with the exception of translucent glass (which suffers from alpha sorting issues when not "structural").