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This dialog controls the transparency part of the volumetric shader. By default, it affects only the smoke. It can affect the fire as well if the Fire Opacity Mode in the Fire rollout is set to Use Smoke Opacity. 

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

Opacity diagram – This diagram specifies the opacity at given point as a function of the channel selected as the Based on parameter. The selected channel's data range will be highlighted in light blue. – 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.

This sim 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 Simulation rollout of Phoenix.

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

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You can use the following controls in the diagram:

Double click – Creates a new point or changes an existing one.
Left button drag over a point – Moves the point. If several points were selected beforehand, they will move the same amount.
Left button drag over several points – Selects several points.
Middle button drag over the background – Drags the visible area. If the Shift key is pressed, scales the diagram in the corresponding direction.
Mouse wheel – Zooms in/out.
Mouse wheel near the borders of the control – Zooms in/out only along X or only along Y.
Right-click – Displays a drop-down menu where you can add a point, edit or delete a selected point, and fit the entire diagram into the view. If multiple points are selected, they can be edited simultaneously. Multiple selected points can also be scaled and flipped with the Scale Selection option as seen below.

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