[vtkusers] where is type VTKRayCastVolumeInfo defined?

[Wolfram H Volpi]

I would like to add "InterpolationType VTK_CUBIC_INTERPOLATION" to
type VTKRayCastVolumeInfo.

Where is VTKRayCastVolumeInfo defined?

What does it mean when "VTK" is in caps.

Thank you,
Wolfv.