The Selective Gaussian Blur filter performs a mathematical region-based selection of the image in small chunks, and determines the level of detail within that chunk. After this it applies a Gaussian-based blur to it. Selective Gaussian Blur can be very processor intensive, but produces very controlled blurring.
Selective Gaussian Blur Settings
The Blur Radius setting affects the maximum number of pixels considered for blurring. The higher the setting, the higher the number of pixels that will be included in the region analysis. Be aware that a higher setting will take considerably longer to compute.
The Max Delta slider affects the level of detail that will be blurred. A higher setting here will produce more smoothing of the pixels in the radius.
A common use for the Selective Gaussian Blur filter is smoothing areas affected by populations of JPEG artifacts, or bad pixelization distortions.
Further information regarding blurring can be found on the Gaussian Blur - RLE and Gaussian Blur - IIR pages.