I think Roger is talking about validation methods that you put inside a backing bean, like shown here:
http://www.ibm.com/developerworks/library/j-jsf3/#N105E7
If you want to go berserk, you could also create a custom validator that conditionally checks required fields.
That way, you'll get something like this:
<h:input... id="theCheckBox" />
<h:input...>
<foo:validateRequiredIfChecked for="#{theCheckBox}" />
</h:input>
Or to make it more reusable:
<h:input... value="#{myBackingBean.someBoolean}" />
<h:input...>
<foo:validateRequired if="#{myBackingBean.someBoolean}" />
</h:input>
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