It's equivalent to the OpenImageDenoise prefiltering option in Cycles.
See D12043.
Prefilter modes:
- None: No prefiltering, use when guiding passes are noise-free.
- Fast: Denoise image and guiding passes together. Improves quality when
guiding passes are noisy using least amount of extra processing time.
- Accurate: Prefilter noisy guiding passes before denoising image.
Improves quality when guiding passes are noisy using extra
processing time.
Reviewed By: #compositing, jbakker, sergey
Differential Revision: https://developer.blender.org/D12342