Specify whether to show the list of the substitution group heads, which this element is member of.
(In other words, these are the elements that this element may substitute for.)
<p>
When <b><code>false</code></b> (unselected), only the number of the substitutable elements
will be printed along with the link to the <i>"Substitutes for elements"</i> section
in the Element Documentation (which, when specified, shows the same list).
<p>
<b>See Also Parameter:</b>
<dl><dd>
<i>"Details | Component Documentation | Lists Of Related Components | Substitutable Elements"</i> 
</dd></dl>