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.
