This is a pass setup I came up with to address a question posted here.
Basically, the query was how to remove individual elements from a pass but still have their reflection appear on other objects. So, in the image above the blue sphere is rendered in the reflections of the grey sphere, but is invisible in the pass.
Basic Setup: All the materials in scene should have a raytype switch plugging into the material node and the rest of the material tree plugging into all ports of the raytype node.
Then in your new pass partition you add an overide property and add the material-Ray_Type-eye parameter to it.
Press 7 with the overide selected and plug a constant shader with 0 for everything including Alpha into the overide node.
You can then drag and drop any objects you like into the "hide" partition and have them not rendered in primary but still appearing in reflections without having to tweak all their rendertrees seperately.
Of course, this still leaves the guy who asked the question with 70 individual passes, so a setup using renderchannels must be preferable:
For every single material you can add a Store Color in channel node between the material node and the material tree and create a new render channel with a name like "blue". Then in the render region pick the channel.
In render manager > pass > Output - add all the new channels to the Render Channels Output list. The advantage is, of course that each individual renderchannel image gets rendered during a single pass.
With a combination of both methods you can break down the scene into its seperate parts by rendering a single multi-channel pass and then create passes as needed to isolate and re-render material changes as required.