The export tab is used for when you want to process a VDB sequence that has been created in Houdini.  The HDA features a number of VDB processing nodes that gives the final render it's look, however if you want to render your VDBs in another DCC such as Maya, you should run your sequences through the HDA before caching out to a VDB frame sequence.

This will ensure that renders match exactly whether rendered with the HDA or using the PxrVolume Pyro Presets.

-- @hope What should these params be set to ?  All ticked on ? --

Hope Edits:

By default, these options will be turned off for full functional use in Houdini. When exporting to another software for rendering using the volume shaders from the RenderMan Preset Manager, check all of these options on to remove extra attributes used for Houdini and prep the volumes from your simulation for OpenVDB sequence export.

When the Export Prepped Sim Fields parameter is checked on, this will skip generating the diffuse and emission color fields because these fields will be generated in the preset shaders in the other software instead.

Optionally, if you would like to use the volumes fields you've generated from the KaBoom Box HDA, leave this option unchecked, and then in the shader applied in the other third-party software, check on 'Use Binding Field Only' in the Smoke, Scatter and/or Fire tabs and enter the string name of your fields in the Bindings tab. See section for (Eugene Riecansky ENTER LINK FOR SECTION - explaining loading VDB and applying preset shader in other software) for more comprehensive instructions on how to do this.

Utilizing the functions of the KaBoom Box HDA is the recommended workflow for shading if the user would like to take advantage of VEX/VDB processes that are native to Houdini SOPs networks. It can also be a more desirable workflow for look-developing using the viewport. Caching the shading calculation data into a VDB can also save render time for high resolution or data heavy volumes.

Eugene Riecansky Section here will a walkthrough from Houdini to Maya for example might be useful for users