  What most people don't know is that this feature actually went
in incomplete. I've had grand plans to finish this, but it's a
biggish project and I've simply never had the time to do it.
What I mean by incomplete is that the underlying framework of
this feature actually involves a process internally known as a
'rendering pipeline' (or just 'pipeline'). This pipeline
contains a number of rendering possibilities and it chooses one
based on available information. If you've used Page Manager
extensively, you've seen this before. It's called a Variant, and
it lets you pick the page that's actually displayed based upon
selection criteria. Typically this involves things like user
access, or node type, or taxonomy term type, or other, less-used
items such as user language.  In most places you can put a
panel, if you have the Panels IPE module there is a radio
selector that lets you pick the standard renderer, or the IPE
renderer. In reality, both of these are pipeline selectors. The
standard renderer always select