The d4p_display-equation is for block equations that should be numbered. If the equation doesn't need to be numbered, then you should use d4p_eqn_block.
This is a specialization of figure. Attention should be paid to your output processes. If they work on an element's class value, then they will see these as regular figures and will not treat them separately, which is probably not the desired result.
+ topic/fig d4p-math-d/d4p_display-equation