Be warned that the evaluation of a shader network for volume rendering is much more expensive than for surface shading, because the shader network is called many times per ray, once per ray march sample. So, in a production environment, it's much better to have a single shader doing all the work, instead of relying on potentially expensive inputs.
Each component (scattering, attenuation and emission) has a source enum, that can be either "parameter" or "channel". On "parameter", the parameter value is used (e.g. scattering). On "channel", the channel (e.g. scattering_channel) is used, its value coming from the density plugin. In addition, the attenuation's source has a third option, called "scattering", in which case the scattering source (parameter or channel) is used. On selecting an enum value (say "channel" for the scattering source), the unused parameter is hidden from the interface. On selecting "parameter", the channel gets hidden. When only channel names are used, the Volume Collector is very similar in functionality to the Arnold core Density shader. Note that the Volume Collector is also fully compatible with the old volume API.
This is the rate at which light is scattered (or reflected) at a given point. The greater the rate of scattering, the shorter the average distance a ray of light will travel through a volume before being bounced off of its course. Usually, the volume density is connected to this parameter.
A color to tint (multiply to) the scattering.
A scale factor to adjust the scattering.
Henyey-Greenstein Anisotropy coefficient between -1 (full back-scatter) and 1 (full forward-scatter). Default is 0 for an isotropic medium.
Note that values very close to 1.0 (above 0.95) or -1.0 (below -0.95) will produce scattering that is so directional that it will not be very visible from most angles, so such values are not recommended.
The method of defining the volume attenuation characteristics.
The intensity is lost due to absorption events only. The volume is guaranteed to conserve energy so it should work robustly no matter the lighting and GI settings, but can appear overly dark.
The intensity is lost due to either scattering or absorption events. It is possible to obtain non-physically real scattering effects in the volume. More light will be scattered than what is actually attenuated by the volume. This is done by setting the attenuation coefficient to a value that is lower than the scattering coefficient. This can be useful to have more control over the brightness of a volume, but causes energy conservation issues which can become troublesome depending on the lighting setup and GI settings.
A color to tint (multiply to) the attenuation.
The rate at which the intensity of a ray traversing a volume is lost.
Emission is the rate at which a volume emits light at a given point. The light emitted by a volume is visible to GI. It will also be affected by any out-scattering or absorption effects in the volume. A heat (flames) grid would be connected to this parameter.
When not empty, this channel will be used to sample the emission values for the volume instead of the Emission parameter. The sampling tab has additional parameters to control how data is sampled from volume channels.
A color to tint (multiply to) the emission.
A scale factor to adjust the emission.
An object space offset to apply to the volume sampling position when using named channels to fetch the volume data. This is useful to displace the volume data.
You will need to connect a noise texture to the position offset to see any result.
The voxel interpolation to use when sampling the volume data using named channels.