I couldn't understand RCC at all, so after understanding it, 
I decided to explain it in plain ENGLISH *shrug* for all of us,
who wasn't on the RCC design commitee.


From LearnSVG article: 
http://www.learnsvg.com/asv6/rcc/arc.php


<extensionDefs namespace="http://www.example.com/subelements">
  <elementDef name="arc">
    <prototype>
        <path id="drawing_arc"/>
    </prototype>
    <script ev:event="SVGBindEnd" type="text/ecmascript" xlink:href="arc.es"/>
  </elementDef>
</extensionDefs>

What that really do is pretty simple:

<extensionDefs>  
is really just a namespace grouping tag,
all it says is that:
Look down here, if you find this namespace in your SVG code.
Something like  namespace http___www_example_com_subelements { }; in C++


<elementDef> is really just like a switch case 
for all possible RCC tag within that namespace,
if you match that name then do this.
Something like  switch( element ) { case name: { } break; } in C++

<script ev:event="SVGBindEnd" type="text/ecmascript" xlink:href="arc.es"/>

means oncreate="" load this script 'arc.es' and execute it using XML-Event hacks.

<prototype></prototype>  

is whatever should be copy-pasted instead of the <pi:arc>
it will also be modified by the "oncreate" script,
to match whatever parameter is passed using XML attributes.

So all this verbose syntax could be simply replaced by something much easier to read:

<prototype xmlns="http://www.example.com/subelements" name="arc">
  <script ev:event="SVGBindEnd" type="text/ecmascript" xlink:href="arc.es"/>
  <path id="drawing_arc"/>
</prototype>

or even simpler:

<prototype xmlns="http://www.example.com/subelements" name="arc" oncreate='createArc(evt)'>
  <script type="text/ecmascript" xlink:href="arc.es"/>
  <path id="drawing_arc"/>
</prototype>


but that would have been to simple, so we use the RCC recommandation syntax instead.

Have fun!
Fred.