The new architecture is starting to gel in my brain.
Came across this today.
Here:
http://kb.x-cart.com/display/XDD/Creating+sidebar+menu+in+customer+area the tpl file is set in the viewer class
Here:
http://xcartguru.com/tutorials/x-cart-5-tutorials/building-a-custom-skin-in-x-cart-5/ the tpl is set in Main.php
Is there a rule for which one to choose?