I try to follow the C-style “Only pay for what you use” mentality. The PRT Volume exposes a list of channels it can fill, and only fills those if an up-stream operation is asking for that channel. If you write a KCM that asks for the Normal channel it is going to fill it in. Also the renderer might request a channel, for motion blur, phong shading, etc.
Its not too smart though, since it will fill the Normal channel in even if an up-stream operation clobbers it with a non-dependent value.
The strategy our objects take is to fill any channel (that the “current” operation has a useful value for) if somewhere an operation closer to the renderer requests it. If you enable Phong shading, and have a KCM that writes to the Normal channel from a texmap, the PRT Volume with dutifully fill in the Normal from the volume even though it gets overwritten by the KCM.
Did that answer the question?
Example of how the channels get figured out:
From bottom to top:
From top to bottom:
3. Renderer requests channels X, Y, and Z
4. KCM sees request for channel Y, so it enables writing to channel Y
5. PRT Volume sees request for channel Y, so it enables writing to channel Y