d4p_MathML is for MathML content.
d4p_MathML is a specialization of foreign. Processors should output the MathML in a suitable fashion. Note the difference in the element name as compared to the version in the d4p_formattingDomain. In this domain, there is an underscore after the "d4p".
+ topic/foreign d4p-math-d/d4p_MathML