sub tag is used to display subscript text. It should not be used to
markup mathematical expressions; instead, use
Content MathML to create expressions with subscripts.
This CNXML:
<para id="directionse">
The northern tip of the lake, or
Tip<sub>North</sub>,
is 10 miles south of the campsite.
</para>
Results in this display:
The northern tip of the lake, or TipNorth, is 10 miles south of the campsite.