For a single equation:
Use equation-inline and equation-block elements to represent a single equation.
For inline equations use equation-inline. Inline equations allow a smooth flow of the sentence that contains the inline equation. Typically, inline equations are not numbered. (For inline equations, numbering could create ambiguity if the number was or was not part of the equation itself.)
To present an equation outside of the sentence flow, use, equation-block. Block equations may be numbered (numbering is optional). [Inline equations are not numbered largely because it could be visually ambiguous as to whether a and it would also disrupt the flow of the sentence that contains the inline equation.]
For multiple equations, or for equations with a caption:
Use the equation-figure element to represents a “display” equation that contains one more distinct equations. The equation-figure element may be numbered and may have a caption. The equation-figure element is a specialization of <fig>. With an equation-figure element, individual equations can have their own numbering.