I suppose the UV Channel mapping at the end of the MxVolumetric object settings lets you add UVs based on different grids contained in the vdb file. The 3dsmax doc on this extension is too sparse I agree, but lets say your vdb file contained among other grids, one grid named "temperature", you write that in the Additional grids text field (how to find out how many grids a vdb file has, and what their names are is explained in the main vdb docs). And then in the UV Channel Mapping section, you would add a Temperature UV map. I suppose it wouldn't matter if you add a "Temperature" or a "Density" or an "Age" UV map in this case because the values contained in a temperature grid in the vdb would be of scalar type, and there are several scalar type UV maps you can choose from, among them Density, Temperature, Age. The other type of data a vdb grid can contain is of type vector, and that would be vorticity for example.
The only thing I guess you need to get right is in what order you add them to that UV stack, based on in what order you add the names in the "Additional grids". Maybe your vdb file contains both a temperature grid, and a particle age grid. If you wanted to use both of these grids to generate UVs, you would write: temperature,age in the Additional grids field, and then add two UVs in the UV Channel Mapping stack: Temperature, Age. Now in your Maxwell material you can add maps and choose if you want to use channel 1 or channel 2 for it since the exported volumetric would now contain two UV channels.
The reason you also need to specify the name of the additional grids you want to use is that the name of those grids can have arbitrary names, depending on what name the person who created the file chose. You can have "temp" or "t" or "temperature" or "machupicchu"....so the plugin can't know what the names of the grids are, you need to specify that. And then add the corresponding UV map types (scalar or vector type) to use that grid data to drive the UVs.
This is all speculation, but it makes sense to me
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