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• Position is in Voxel space. It's in the coordinate system of the simulation grid the particles belong to. This way, if a particle is inside a grid 100 voxels wide, then its coordinates would range from 0-100.
  
• Velocity is in Voxels/sec, so it depends on the grid size and resolution, and its length usually goes from 0 to several hundred. In Phoenix, the length of the Velocity is referred to as Speed.
  
• The ID of a particle is its unique born index. Each particle has a different integer ID - the IDs start from 0 and are increased for each new particle in the simulation.
• The particle Age is in seconds. It can not be negative, starts from 0 when a particle is born, and increases over time.
• The Phoenix particle Size channel represents the radius of the particle. If the particle is a sphere, such as a foam bubble, then this is just half the size of the sphere end to end. Phoenix particle sizes are kept as a fraction of the voxel size of the simulation grid they belong to. If a simulation has no particle Size channel exported to the Phoenix cache files, then the particles are assumed to have a radius equal to the size of one voxel.
• Liquid particles in Phoenix have a Viscosity channel - it denotes the viscosity of each liquid particle and usually ranges from 0-1, though in some simulations it can go above 1 in order to represent very dense liquids.