* Location: Basically the same as the location from Object Info node for object instances on particles, but in principle there could be additional offsets for dupli objects, so included for completeness.
* Size: Single float scale of the particle. Also directly translates to object scale for current dupli objects, but handy to have as a single float to start with instead of a scale vector (currently not even exposed in Object Info).
* Rotation: This is a quaternion, which are not yet supported by Cycles nodes. The float4 is copied to internal Cycles data and stored in the particles texture data, but the node doesn't have a socket for it yet and the data is not yet written to the stack. Code is just commented out so could be enabled quickly if/when rotation support is added to cycles.
* Velocity: Linear velocity vector of particles.
* Angular Velocity: Angular velocity around principle axes.
The texture data is currently packed tightly into the particles texture, which saves a few bytes, but requires an additional texture lookup for some vector attributes which spread over two float4s. Could also add another float4 to particle size to avoid this.
The particle data used by the Particle Info node was stored in cycles as a list in each object. This is a problem when the particle emitter mesh is hidden: Objects in cycles are only intended as instances of renderable meshes, so when hiding the emitter mesh the particle data doesn't get stored either. Also the particle data can potentially be copied to multiple instances of the same object, which is a waste of texture space.
The solution in this patch is to make a completely separate list of particle systems in the Cycles scene data. This way the particle data can be generated even when the emitter object itself is not visible.
