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Based on | trSource – Allows you to select how the smoke opacity is computed. By default, this is set to the Simple Smoke option. It could also be based on a texture map, or read from the cache files - in that case, the corresponding grid channel must be present in the cache files and remapped from the Input roll-out using the 3rd Party Channel Mappings dialog.

SimpleSmoke – Uses directly the Smoke channel as opacity, without remapping it through the Opacity Curve. The opacity is controlled by the Simple Smoke Opacity. This mode is the same as setting Based on to Smoke and creating a straight line in the Opacity diagram from coordinates [0; 0] to [1; Simple Smoke Opacity].
Temperature – Use the temperature channel and remap it through the Opacity Curve to get the rendered opacity. Can be used to produce smoke without needing the smoke channel in the simulation.
Smoke – Use the smoke channel and remap it through the Opacity Curve to get the rendered opacity.
Speed – Use the magnitude of the velocity channel and remap it through the Opacity Curve to get the rendered opacity.
Texture – Use a texture map plugged in the Texture slot as opacity, without remapping it through the Opacity Curve.
Fuel – Use the fuel channel and remap it through the Opacity Curve to get the rendered opacity.

Texture | trTexture – If the Based on parameter is set to Texture, or if Modulate is enabled, this slot specifies the texture to be used.

Modulate | trModulate – When enabled and Based on is not set to Texture, the Based on channel is multiplied by the texture in the Texture slot.

Reset to Defaults – Resets the settings to their default values.

Absorption  – Controls the color of the volumetric shadows and the tint for the objects seen through the volume. Brighter colors make the volume more transparent, while darker colors make it more opaque (denser).

• Constant Color – Specifies the color that will be used for the absorption effect.
• Texture – When a texture is connected in the slot the absorption constant color will be ignored and the texture color will be used instead.

Simple Smoke Opacity | trSimpleSmFactor – When the Based on parameter is set to Simple smoke, this value is used as opacity multiplier.

Minimum Visible Opacity | trSkipShLevel – If the opacity of a voxel is below this specified value, the voxel will not be shaded at all. Increase this value to speed up rendering. Decrease it if parts of the smoke are too transparent and start disappearing in the render.

Scale Opacity by Scene Units | trAlphaUnitScale – When enabled, the opacity per unit length will remain constant when changing the Grid resolution. Therefore, the larger the unit scale is, the denser the smoke will appear in renders and vice versa. This option is Off for imported Field3D and OpenVDB cache files. Note that this option also affects Fire in Use Own Opacity mode. It does not take effect if there is no loaded cache file.

Optimize Big Volumetric Grids | trOptimizeVolumetrics – Helps render scenes with caches above 25 Million cells faster. This option requires additional grid memory during rendering, so you could disable it if you don't have enough RAM. If using Progressive rendering or there are a lot of Volume Grids in the scene, and you want your render to start without a pre-pass, you might want to disable this option.  

Opacity diagram – When the Based on option is set to a grid channel, this curve remaps grid simulation data to smoke opacity. The sim data channel is laid out horizontally along the X axis in the opacity curve. The opacity is mapped vertically on the Y axis.

Simulation data has different ranges, depending on the solver it came from - Phoenix Smoke usually goes from 0-1; Fuel goes from 0-1; Temperature is in Kelvins, so it goes from 300-2000, or a few thousand; Speed is in Voxels/sec, so it depends on the grid size and resolution, but usually goes from 0 to several hundred. Data from other solvers often differs a lot from Phoenix ranges. The channel data range is highlighted in the curve with a light-blue range which can change as you scroll the timeline as the temperature in a cache file rises, smoke dissipates, speed changes, etc., so keep in mind that the data range for one frame might not be representative for the entire cache sequence. You can also check the data ranges for the currently loaded cache file under the Inout roll-out of the V-Ray Volume Grid.