Similar to the Mesh Renderer, the difference lies in the Grid Size and the Positions Per Cell (PPC) parameters. The Grid Size is the number of regular cells per dimension of the cubic volume that is used for the rendering. The total number of cells in the cube is Grid Size^3.
In order so save memory, only cells are activated that are actually populated by the particles of the object. If an object includes a vacuum object, the cells that are in this vacuum are deactivated.
In some circumstances it aliasing effects may occur due to the presence of two different regular grids (the 3D volume grid and the 2D image grid). This can be avoided by choosing the Grid Size in powers of 2, similar to the 2D image size).
The PPC parameter is the number of samples of physical distributions in space (like density, temperature, velocity, etc.) that are taken in every active cell of the 3D grid. In each cell the average value is then used for rendering. This may improve accuracy of the result for quickly varying fields. Note, however, that using values larger than 1, will also increase the rendering time.