roman designates text should not be given any empohasis, such as bold or italic. It is useful for when a parent block element assigns bold, italic, etc., such as a heading. In such cases, the use of roman can actually provide emphasis to the content by making it standout.
roman is a specialization of ph.
+ topic/ph d4p-formatting-d/roman
<p outputclass="bold">This text is bold, <roman>but this is roman.</roman></p>