* 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.
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